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# coding: utf-8
# pylint: disable=arguments-differ
""" losses for training neural networks """
from __future__ import absolute_import
__all__ = ['Loss', 'L2Loss', 'L1Loss',
'SigmoidBinaryCrossEntropyLoss', 'SigmoidBCELoss',
'SoftmaxCrossEntropyLoss', 'SoftmaxCELoss',
'KLDivLoss', 'CTCLoss', 'HuberLoss', 'HingeLoss',
'SquaredHingeLoss', 'LogisticLoss', 'TripletLoss']
from .. import ndarray
from ..base import numeric_types
from .block import HybridBlock
def _apply_weighting(F, loss, weight=None, sample_weight=None):
"""Apply weighting to loss.
Parameters
----------
loss : Symbol
The loss to be weighted.
weight : float or None
Global scalar weight for loss.
sample_weight : Symbol or None
Per sample weighting. Must be broadcastable to
the same shape as loss. For example, if loss has
shape (64, 10) and you want to weight each sample
in the batch separately, `sample_weight` should have
shape (64, 1).
Returns
-------
loss : Symbol
Weighted loss
"""
if sample_weight is not None:
loss = F.broadcast_mul(loss, sample_weight)
if weight is not None:
assert isinstance(weight, numeric_types), "weight must be a number"
loss = loss * weight
return loss
def _reshape_like(F, x, y):
"""Reshapes x to the same shape as y."""
return x.reshape(y.shape) if F is ndarray else F.reshape_like(x, y)
[docs]class Loss(HybridBlock):
"""Base class for loss.
Parameters
----------
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
"""
def __init__(self, weight, batch_axis, **kwargs):
super(Loss, self).__init__(**kwargs)
self._weight = weight
self._batch_axis = batch_axis
def __repr__(self):
s = '{name}(batch_axis={_batch_axis}, w={_weight})'
return s.format(name=self.__class__.__name__, **self.__dict__)
[docs] def hybrid_forward(self, F, x, *args, **kwargs):
"""Overrides to construct symbolic graph for this `Block`.
Parameters
----------
x : Symbol or NDArray
The first input tensor.
*args : list of Symbol or list of NDArray
Additional input tensors.
"""
# pylint: disable= invalid-name
raise NotImplementedError
[docs]class L2Loss(Loss):
r"""Calculates the mean squared error between `pred` and `label`.
.. math:: L = \frac{1}{2} \sum_i \vert {pred}_i - {label}_i \vert^2.
`pred` and `label` can have arbitrary shape as long as they have the same
number of elements.
Parameters
----------
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: prediction tensor with arbitrary shape
- **label**: target tensor with the same size as pred.
- **sample_weight**: element-wise weighting tensor. Must be broadcastable
to the same shape as pred. For example, if pred has shape (64, 10)
and you want to weigh each sample in the batch separately,
sample_weight should have shape (64, 1).
Outputs:
- **loss**: loss tensor with shape (batch_size,). Dimenions other than
batch_axis are averaged out.
"""
def __init__(self, weight=1., batch_axis=0, **kwargs):
super(L2Loss, self).__init__(weight, batch_axis, **kwargs)
def hybrid_forward(self, F, pred, label, sample_weight=None):
label = _reshape_like(F, label, pred)
loss = F.square(pred - label)
loss = _apply_weighting(F, loss, self._weight/2, sample_weight)
return F.mean(loss, axis=self._batch_axis, exclude=True)
[docs]class L1Loss(Loss):
r"""Calculates the mean absolute error between `pred` and `label`.
.. math:: L = \sum_i \vert {pred}_i - {label}_i \vert.
`pred` and `label` can have arbitrary shape as long as they have the same
number of elements.
Parameters
----------
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: prediction tensor with arbitrary shape
- **label**: target tensor with the same size as pred.
- **sample_weight**: element-wise weighting tensor. Must be broadcastable
to the same shape as pred. For example, if pred has shape (64, 10)
and you want to weigh each sample in the batch separately,
sample_weight should have shape (64, 1).
Outputs:
- **loss**: loss tensor with shape (batch_size,). Dimenions other than
batch_axis are averaged out.
"""
def __init__(self, weight=None, batch_axis=0, **kwargs):
super(L1Loss, self).__init__(weight, batch_axis, **kwargs)
def hybrid_forward(self, F, pred, label, sample_weight=None):
label = _reshape_like(F, label, pred)
loss = F.abs(pred - label)
loss = _apply_weighting(F, loss, self._weight, sample_weight)
return F.mean(loss, axis=self._batch_axis, exclude=True)
[docs]class SigmoidBinaryCrossEntropyLoss(Loss):
r"""The cross-entropy loss for binary classification. (alias: SigmoidBCELoss)
BCE loss is useful when training logistic regression. If `from_sigmoid`
is False (default), this loss computes:
.. math::
prob = \frac{1}{1 + \exp(-{pred})}
L = - \sum_i {label}_i * \log({prob}_i) +
(1 - {label}_i) * \log(1 - {prob}_i)
If `from_sigmoid` is True, this loss computes:
.. math::
L = - \sum_i {label}_i * \log({pred}_i) +
(1 - {label}_i) * \log(1 - {pred}_i)
`pred` and `label` can have arbitrary shape as long as they have the same
number of elements.
Parameters
----------
from_sigmoid : bool, default is `False`
Whether the input is from the output of sigmoid. Set this to false will make
the loss calculate sigmoid and BCE together, which is more numerically
stable through log-sum-exp trick.
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: prediction tensor with arbitrary shape
- **label**: target tensor with values in range `[0, 1]`. Must have the
same size as `pred`.
- **sample_weight**: element-wise weighting tensor. Must be broadcastable
to the same shape as pred. For example, if pred has shape (64, 10)
and you want to weigh each sample in the batch separately,
sample_weight should have shape (64, 1).
Outputs:
- **loss**: loss tensor with shape (batch_size,). Dimenions other than
batch_axis are averaged out.
"""
def __init__(self, from_sigmoid=False, weight=None, batch_axis=0, **kwargs):
super(SigmoidBinaryCrossEntropyLoss, self).__init__(weight, batch_axis, **kwargs)
self._from_sigmoid = from_sigmoid
def hybrid_forward(self, F, pred, label, sample_weight=None):
label = _reshape_like(F, label, pred)
if not self._from_sigmoid:
max_val = F.relu(-pred)
loss = pred - pred*label + max_val + F.log(F.exp(-max_val)+F.exp(-pred-max_val))
else:
loss = -(F.log(pred+1e-12)*label + F.log(1.-pred+1e-12)*(1.-label))
loss = _apply_weighting(F, loss, self._weight, sample_weight)
return F.mean(loss, axis=self._batch_axis, exclude=True)
SigmoidBCELoss = SigmoidBinaryCrossEntropyLoss
[docs]class SoftmaxCrossEntropyLoss(Loss):
r"""Computes the softmax cross entropy loss. (alias: SoftmaxCELoss)
If `sparse_label` is `True` (default), label should contain integer
category indicators:
.. math::
\DeclareMathOperator{softmax}{softmax}
p = \softmax({pred})
L = -\sum_i \log p_{i,{label}_i}
`label`'s shape should be `pred`'s shape with the `axis` dimension removed.
i.e. for `pred` with shape (1,2,3,4) and `axis = 2`, `label`'s shape should
be (1,2,4).
If `sparse_label` is `False`, `label` should contain probability distribution
and `label`'s shape should be the same with `pred`:
.. math::
p = \softmax({pred})
L = -\sum_i \sum_j {label}_j \log p_{ij}
Parameters
----------
axis : int, default -1
The axis to sum over when computing softmax and entropy.
sparse_label : bool, default True
Whether label is an integer array instead of probability distribution.
from_logits : bool, default False
Whether input is a log probability (usually from log_softmax) instead
of unnormalized numbers.
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: the prediction tensor, where the `batch_axis` dimension
ranges over batch size and `axis` dimension ranges over the number
of classes.
- **label**: the truth tensor. When `sparse_label` is True, `label`'s
shape should be `pred`'s shape with the `axis` dimension removed.
i.e. for `pred` with shape (1,2,3,4) and `axis = 2`, `label`'s shape
should be (1,2,4) and values should be integers between 0 and 2. If
`sparse_label` is False, `label`'s shape must be the same as `pred`
and values should be floats in the range `[0, 1]`.
- **sample_weight**: element-wise weighting tensor. Must be broadcastable
to the same shape as label. For example, if label has shape (64, 10)
and you want to weigh each sample in the batch separately,
sample_weight should have shape (64, 1).
Outputs:
- **loss**: loss tensor with shape (batch_size,). Dimenions other than
batch_axis are averaged out.
"""
def __init__(self, axis=-1, sparse_label=True, from_logits=False, weight=None,
batch_axis=0, **kwargs):
super(SoftmaxCrossEntropyLoss, self).__init__(weight, batch_axis, **kwargs)
self._axis = axis
self._sparse_label = sparse_label
self._from_logits = from_logits
def hybrid_forward(self, F, pred, label, sample_weight=None):
if not self._from_logits:
pred = F.log_softmax(pred, self._axis)
if self._sparse_label:
loss = -F.pick(pred, label, axis=self._axis, keepdims=True)
else:
label = _reshape_like(F, label, pred)
loss = -F.sum(pred*label, axis=self._axis, keepdims=True)
loss = _apply_weighting(F, loss, self._weight, sample_weight)
return F.mean(loss, axis=self._batch_axis, exclude=True)
SoftmaxCELoss = SoftmaxCrossEntropyLoss
[docs]class KLDivLoss(Loss):
r"""The Kullback-Leibler divergence loss.
KL divergence measures the distance between contiguous distributions. It
can be used to minimize information loss when approximating a distribution.
If `from_logits` is True (default), loss is defined as:
.. math::
L = \sum_i {label}_i * \big[\log({label}_i) - {pred}_i\big]
If `from_logits` is False, loss is defined as:
.. math::
\DeclareMathOperator{softmax}{softmax}
prob = \softmax({pred})
L = \sum_i {label}_i * \big[\log({label}_i) - log({pred}_i)\big]
`pred` and `label` can have arbitrary shape as long as they have the same
number of elements.
Parameters
----------
from_logits : bool, default is `True`
Whether the input is log probability (usually from log_softmax) instead
of unnormalized numbers.
axis : int, default -1
The dimension along with to compute softmax. Only used when `from_logits`
is False.
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: prediction tensor with arbitrary shape. If `from_logits` is
True, `pred` should be log probabilities. Otherwise, it should be
unnormalized predictions, i.e. from a dense layer.
- **label**: truth tensor with values in range `(0, 1)`. Must have
the same size as `pred`.
- **sample_weight**: element-wise weighting tensor. Must be broadcastable
to the same shape as pred. For example, if pred has shape (64, 10)
and you want to weigh each sample in the batch separately,
sample_weight should have shape (64, 1).
Outputs:
- **loss**: loss tensor with shape (batch_size,). Dimenions other than
batch_axis are averaged out.
References
----------
`Kullback-Leibler divergence
`_
"""
def __init__(self, from_logits=True, axis=-1, weight=None, batch_axis=0,
**kwargs):
super(KLDivLoss, self).__init__(weight, batch_axis, **kwargs)
self._from_logits = from_logits
self._axis = axis
def hybrid_forward(self, F, pred, label, sample_weight=None):
if not self._from_logits:
pred = F.log_softmax(pred, self._axis)
loss = label * (F.log(label+1e-12) - pred)
loss = _apply_weighting(F, loss, self._weight, sample_weight)
return F.mean(loss, axis=self._batch_axis, exclude=True)
[docs]class CTCLoss(Loss):
r"""Connectionist Temporal Classification Loss.
Parameters
----------
layout : str, default 'NTC'
Layout of prediction tensor. 'N', 'T', 'C' stands for batch size,
sequence length, and alphabet_size respectively.
label_layout : str, default 'NT'
Layout of the labels. 'N', 'T' stands for batch size, and sequence
length respectively.
weight : float or None
Global scalar weight for loss.
Inputs:
- **pred**: unnormalized prediction tensor (before softmax).
Its shape depends on `layout`. If `layout` is 'TNC', pred
should have shape `(sequence_length, batch_size, alphabet_size)`.
Note that in the last dimension, index `alphabet_size-1` is reserved
for internal use as blank label. So `alphabet_size` is one plus the
actual alphabet size.
- **label**: zero-based label tensor. Its shape depends on `label_layout`.
If `label_layout` is 'TN', `label` should have shape
`(label_sequence_length, batch_size)`.
- **pred_lengths**: optional (default None), used for specifying the
length of each entry when different `pred` entries in the same batch
have different lengths. `pred_lengths` should have shape `(batch_size,)`.
- **label_lengths**: optional (default None), used for specifying the
length of each entry when different `label` entries in the same batch
have different lengths. `label_lengths` should have shape `(batch_size,)`.
Outputs:
- **loss**: output loss has shape `(batch_size,)`.
**Example**: suppose the vocabulary is `[a, b, c]`, and in one batch we
have three sequences 'ba', 'cbb', and 'abac'. We can index the labels as
`{'a': 0, 'b': 1, 'c': 2, blank: 3}`. Then `alphabet_size` should be 4,
where label 3 is reserved for internal use by `CTCLoss`. We then need to
pad each sequence with `-1` to make a rectangular `label` tensor::
[[1, 0, -1, -1],
[2, 1, 1, -1],
[0, 1, 0, 2]]
References
----------
`Connectionist Temporal Classification: Labelling Unsegmented
Sequence Data with Recurrent Neural Networks
`_
"""
def __init__(self, layout='NTC', label_layout='NT', weight=None, **kwargs):
assert layout in ['NTC', 'TNC'],\
"Only 'NTC' and 'TNC' layouts for pred are supported. Got: %s"%layout
assert label_layout in ['NT', 'TN'],\
"Only 'NT' and 'TN' layouts for label are supported. Got: %s"%label_layout
self._layout = layout
self._label_layout = label_layout
batch_axis = label_layout.find('N')
super(CTCLoss, self).__init__(weight, batch_axis, **kwargs)
def hybrid_forward(self, F, pred, label,
pred_lengths=None, label_lengths=None, sample_weight=None):
if self._layout == 'NTC':
pred = F.swapaxes(pred, 0, 1)
if self._batch_axis == 1:
label = F.swapaxes(label, 0, 1)
loss = F.contrib.CTCLoss(pred, label, pred_lengths, label_lengths,
use_data_lengths=pred_lengths is not None,
use_label_lengths=label_lengths is not None,
blank_label='last')
return _apply_weighting(F, loss, self._weight, sample_weight)
[docs]class HuberLoss(Loss):
r"""Calculates smoothed L1 loss that is equal to L1 loss if absolute error
exceeds rho but is equal to L2 loss otherwise. Also called SmoothedL1 loss.
.. math::
L = \sum_i \begin{cases} \frac{1}{2 {rho}} ({pred}_i - {label}_i)^2 &
\text{ if } |{pred}_i - {label}_i| < {rho} \\
|{pred}_i - {label}_i| - \frac{{rho}}{2} &
\text{ otherwise }
\end{cases}
`pred` and `label` can have arbitrary shape as long as they have the same
number of elements.
Parameters
----------
rho : float, default 1
Threshold for trimmed mean estimator.
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: prediction tensor with arbitrary shape
- **label**: target tensor with the same size as pred.
- **sample_weight**: element-wise weighting tensor. Must be broadcastable
to the same shape as pred. For example, if pred has shape (64, 10)
and you want to weigh each sample in the batch separately,
sample_weight should have shape (64, 1).
Outputs:
- **loss**: loss tensor with shape (batch_size,). Dimenions other than
batch_axis are averaged out.
"""
def __init__(self, rho=1, weight=None, batch_axis=0, **kwargs):
super(HuberLoss, self).__init__(weight, batch_axis, **kwargs)
self._rho = rho
def hybrid_forward(self, F, pred, label, sample_weight=None):
label = _reshape_like(F, label, pred)
loss = F.abs(pred - label)
loss = F.where(loss > self._rho, loss - 0.5 * self._rho,
(0.5/self._rho) * F.square(loss))
loss = _apply_weighting(F, loss, self._weight, sample_weight)
return F.mean(loss, axis=self._batch_axis, exclude=True)
[docs]class HingeLoss(Loss):
r"""Calculates the hinge loss function often used in SVMs:
.. math::
L = \sum_i max(0, {margin} - {pred}_i \cdot {label}_i)
where `pred` is the classifier prediction and `label` is the target tensor
containing values -1 or 1. `pred` and `label` must have the same number of
elements.
Parameters
----------
margin : float
The margin in hinge loss. Defaults to 1.0
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: prediction tensor with arbitrary shape.
- **label**: truth tensor with values -1 or 1. Must have the same size
as pred.
- **sample_weight**: element-wise weighting tensor. Must be broadcastable
to the same shape as pred. For example, if pred has shape (64, 10)
and you want to weigh each sample in the batch separately,
sample_weight should have shape (64, 1).
Outputs:
- **loss**: loss tensor with shape (batch_size,). Dimenions other than
batch_axis are averaged out.
"""
def __init__(self, margin=1, weight=None, batch_axis=0, **kwargs):
super(HingeLoss, self).__init__(weight, batch_axis, **kwargs)
self._margin = margin
def hybrid_forward(self, F, pred, label, sample_weight=None):
label = _reshape_like(F, label, pred)
loss = F.relu(self._margin - pred * label)
loss = _apply_weighting(F, loss, self._weight, sample_weight)
return F.mean(loss, axis=self._batch_axis, exclude=True)
[docs]class SquaredHingeLoss(Loss):
r"""Calculates the soft-margin loss function used in SVMs:
.. math::
L = \sum_i max(0, {margin} - {pred}_i \cdot {label}_i)^2
where `pred` is the classifier prediction and `label` is the target tensor
containing values -1 or 1. `pred` and `label` can have arbitrary shape as
long as they have the same number of elements.
Parameters
----------
margin : float
The margin in hinge loss. Defaults to 1.0
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: prediction tensor with arbitrary shape
- **label**: truth tensor with values -1 or 1. Must have the same size
as pred.
- **sample_weight**: element-wise weighting tensor. Must be broadcastable
to the same shape as pred. For example, if pred has shape (64, 10)
and you want to weigh each sample in the batch separately,
sample_weight should have shape (64, 1).
Outputs:
- **loss**: loss tensor with shape (batch_size,). Dimenions other than
batch_axis are averaged out.
"""
def __init__(self, margin=1, weight=None, batch_axis=0, **kwargs):
super(SquaredHingeLoss, self).__init__(weight, batch_axis, **kwargs)
self._margin = margin
def hybrid_forward(self, F, pred, label, sample_weight=None):
label = _reshape_like(F, label, pred)
loss = F.square(F.relu(self._margin - pred * label))
loss = _apply_weighting(F, loss, self._weight, sample_weight)
return F.mean(loss, axis=self._batch_axis, exclude=True)
[docs]class LogisticLoss(Loss):
r"""Calculates the logistic loss (for binary losses only):
.. math::
L = \sum_i \log(1 + \exp(- {pred}_i \cdot {label}_i))
where `pred` is the classifier prediction and `label` is the target tensor
containing values -1 or 1. `pred` and `label` can have arbitrary shape as
long as they have the same number of elements.
Parameters
----------
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: prediction tensor with arbitrary shape.
- **label**: truth tensor with values -1 or 1. Must have the same size
as pred.
- **sample_weight**: element-wise weighting tensor. Must be broadcastable
to the same shape as pred. For example, if pred has shape (64, 10)
and you want to weigh each sample in the batch separately,
sample_weight should have shape (64, 1).
Outputs:
- **loss**: loss tensor with shape (batch_size,). Dimenions other than
batch_axis are averaged out.
"""
def __init__(self, weight=None, batch_axis=0, **kwargs):
super(LogisticLoss, self).__init__(weight, batch_axis, **kwargs)
def hybrid_forward(self, F, pred, label, sample_weight=None):
label = _reshape_like(F, label, pred)
loss = F.log(1.0 + F.exp(-pred * label))
loss = _apply_weighting(F, loss, self._weight, sample_weight)
return F.mean(loss, axis=self._batch_axis, exclude=True)
[docs]class TripletLoss(Loss):
r"""Calculates triplet loss given three input tensors and a positive margin.
Triplet loss measures the relative similarity between prediction, a positive
example and a negative example:
.. math::
L = \sum_i \max(\Vert {pred}_i - {pos_i} \Vert_2^2 -
\Vert {pred}_i - {neg_i} \Vert_2^2 + {margin}, 0)
`pred`, `positive` and `negative` can have arbitrary shape as long as they
have the same number of elements.
Parameters
----------
margin : float
Margin of separation between correct and incorrect pair.
weight : float or None
Global scalar weight for loss.
batch_axis : int, default 0
The axis that represents mini-batch.
Inputs:
- **pred**: prediction tensor with arbitrary shape
- **positive**: positive example tensor with arbitrary shape. Must have
the same size as pred.
- **negative**: negative example tensor with arbitrary shape Must have
the same size as pred.
Outputs:
- **loss**: loss tensor with shape (batch_size,).
"""
def __init__(self, margin=1, weight=None, batch_axis=0, **kwargs):
super(TripletLoss, self).__init__(weight, batch_axis, **kwargs)
self._margin = margin
def hybrid_forward(self, F, pred, positive, negative):
positive = _reshape_like(F, positive, pred)
negative = _reshape_like(F, negative, pred)
loss = F.sum(F.square(pred-positive) - F.square(pred-negative),
axis=self._batch_axis, exclude=True)
loss = F.relu(loss + self._margin)
return _apply_weighting(F, loss, self._weight, None)
