# Copyright 2019 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================
"""Functions and classes related to optimization (weight updates)."""
import re
import tensorflow as tf
[docs]class WarmUp(tf.keras.optimizers.schedules.LearningRateSchedule):
"""Applies a warmup schedule on a given learning rate decay schedule."""
def __init__(
self, initial_learning_rate, decay_schedule_fn, warmup_steps, power=1.0, name=None,
):
super().__init__()
self.initial_learning_rate = initial_learning_rate
self.warmup_steps = warmup_steps
self.power = power
self.decay_schedule_fn = decay_schedule_fn
self.name = name
def __call__(self, step):
with tf.name_scope(self.name or "WarmUp") as name:
# Implements polynomial warmup. i.e., if global_step < warmup_steps, the
# learning rate will be `global_step/num_warmup_steps * init_lr`.
global_step_float = tf.cast(step, tf.float32)
warmup_steps_float = tf.cast(self.warmup_steps, tf.float32)
warmup_percent_done = global_step_float / warmup_steps_float
warmup_learning_rate = self.initial_learning_rate * tf.math.pow(warmup_percent_done, self.power)
return tf.cond(
global_step_float < warmup_steps_float,
lambda: warmup_learning_rate,
lambda: self.decay_schedule_fn(step),
name=name,
)
def get_config(self):
return {
"initial_learning_rate": self.initial_learning_rate,
"decay_schedule_fn": self.decay_schedule_fn,
"warmup_steps": self.warmup_steps,
"power": self.power,
"name": self.name,
}
[docs]def create_optimizer(init_lr, num_train_steps, num_warmup_steps, end_lr=0.0, optimizer_type="adamw"):
"""Creates an optimizer with learning rate schedule."""
# Implements linear decay of the learning rate.
lr_schedule = tf.keras.optimizers.schedules.PolynomialDecay(
initial_learning_rate=init_lr, decay_steps=num_train_steps, end_learning_rate=end_lr,
)
if num_warmup_steps:
lr_schedule = WarmUp(
initial_learning_rate=init_lr, decay_schedule_fn=lr_schedule, warmup_steps=num_warmup_steps,
)
optimizer = AdamWeightDecay(
learning_rate=lr_schedule,
weight_decay_rate=0.01,
beta_1=0.9,
beta_2=0.999,
epsilon=1e-6,
exclude_from_weight_decay=["layer_norm", "bias"],
)
return optimizer
[docs]class AdamWeightDecay(tf.keras.optimizers.Adam):
"""Adam enables L2 weight decay and clip_by_global_norm on gradients.
Just adding the square of the weights to the loss function is *not* the
correct way of using L2 regularization/weight decay with Adam, since that will
interact with the m and v parameters in strange ways.
Instead we want ot decay the weights in a manner that doesn't interact with
the m/v parameters. This is equivalent to adding the square of the weights to
the loss with plain (non-momentum) SGD.
"""
def __init__(
self,
learning_rate=0.001,
beta_1=0.9,
beta_2=0.999,
epsilon=1e-7,
amsgrad=False,
weight_decay_rate=0.0,
include_in_weight_decay=None,
exclude_from_weight_decay=None,
name="AdamWeightDecay",
**kwargs
):
super().__init__(learning_rate, beta_1, beta_2, epsilon, amsgrad, name, **kwargs)
self.weight_decay_rate = weight_decay_rate
self._include_in_weight_decay = include_in_weight_decay
self._exclude_from_weight_decay = exclude_from_weight_decay
[docs] @classmethod
def from_config(cls, config):
"""Creates an optimizer from its config with WarmUp custom object."""
custom_objects = {"WarmUp": WarmUp}
return super(AdamWeightDecay, cls).from_config(config, custom_objects=custom_objects)
def _prepare_local(self, var_device, var_dtype, apply_state):
super(AdamWeightDecay, self)._prepare_local(var_device, var_dtype, apply_state)
apply_state[(var_device, var_dtype)]["weight_decay_rate"] = tf.constant(
self.weight_decay_rate, name="adam_weight_decay_rate"
)
def _decay_weights_op(self, var, learning_rate, apply_state):
do_decay = self._do_use_weight_decay(var.name)
if do_decay:
return var.assign_sub(
learning_rate * var * apply_state[(var.device, var.dtype.base_dtype)]["weight_decay_rate"],
use_locking=self._use_locking,
)
return tf.no_op()
[docs] def apply_gradients(self, grads_and_vars, name=None):
grads, tvars = list(zip(*grads_and_vars))
return super(AdamWeightDecay, self).apply_gradients(zip(grads, tvars), name=name,)
def _get_lr(self, var_device, var_dtype, apply_state):
"""Retrieves the learning rate with the given state."""
if apply_state is None:
return self._decayed_lr_t[var_dtype], {}
apply_state = apply_state or {}
coefficients = apply_state.get((var_device, var_dtype))
if coefficients is None:
coefficients = self._fallback_apply_state(var_device, var_dtype)
apply_state[(var_device, var_dtype)] = coefficients
return coefficients["lr_t"], dict(apply_state=apply_state)
def _resource_apply_dense(self, grad, var, apply_state=None):
lr_t, kwargs = self._get_lr(var.device, var.dtype.base_dtype, apply_state)
decay = self._decay_weights_op(var, lr_t, apply_state)
with tf.control_dependencies([decay]):
return super(AdamWeightDecay, self)._resource_apply_dense(grad, var, **kwargs)
def _resource_apply_sparse(self, grad, var, indices, apply_state=None):
lr_t, kwargs = self._get_lr(var.device, var.dtype.base_dtype, apply_state)
decay = self._decay_weights_op(var, lr_t, apply_state)
with tf.control_dependencies([decay]):
return super(AdamWeightDecay, self)._resource_apply_sparse(grad, var, indices, **kwargs)
[docs] def get_config(self):
config = super().get_config()
config.update({"weight_decay_rate": self.weight_decay_rate})
return config
def _do_use_weight_decay(self, param_name):
"""Whether to use L2 weight decay for `param_name`."""
if self.weight_decay_rate == 0:
return False
if self._include_in_weight_decay:
for r in self._include_in_weight_decay:
if re.search(r, param_name) is not None:
return True
if self._exclude_from_weight_decay:
for r in self._exclude_from_weight_decay:
if re.search(r, param_name) is not None:
return False
return True
# Extracted from https://github.com/OpenNMT/OpenNMT-tf/blob/master/opennmt/optimizers/utils.py
[docs]class GradientAccumulator(object):
"""Gradient accumulation utility.
When used with a distribution strategy, the accumulator should be called in a
replica context. Gradients will be accumulated locally on each replica and
without synchronization. Users should then call ``.gradients``, scale the
gradients if required, and pass the result to ``apply_gradients``.
"""
# We use the ON_READ synchronization policy so that no synchronization is
# performed on assignment. To get the value, we call .value() which returns the
# value on the current replica without synchronization.
def __init__(self):
"""Initializes the accumulator."""
self._gradients = []
self._accum_steps = None
@property
def step(self):
"""Number of accumulated steps."""
if self._accum_steps is None:
self._accum_steps = tf.Variable(
tf.constant(0, dtype=tf.int64),
trainable=False,
synchronization=tf.VariableSynchronization.ON_READ,
aggregation=tf.VariableAggregation.ONLY_FIRST_REPLICA,
)
return self._accum_steps.value()
@property
def gradients(self):
"""The accumulated gradients on the current replica."""
if not self._gradients:
raise ValueError("The accumulator should be called first to initialize the gradients")
return list(gradient.value() for gradient in self._gradients)
def __call__(self, gradients):
"""Accumulates :obj:`gradients` on the current replica."""
if not self._gradients:
_ = self.step # Create the step variable.
self._gradients.extend(
[
tf.Variable(
tf.zeros_like(gradient),
trainable=False,
synchronization=tf.VariableSynchronization.ON_READ,
aggregation=tf.VariableAggregation.ONLY_FIRST_REPLICA,
)
for gradient in gradients
]
)
if len(gradients) != len(self._gradients):
raise ValueError("Expected %s gradients, but got %d" % (len(self._gradients), len(gradients)))
for accum_gradient, gradient in zip(self._gradients, gradients):
accum_gradient.assign_add(gradient)
self._accum_steps.assign_add(1)
def reset(self):
"""Resets the accumulated gradients on the current replica."""
if not self._gradients:
return
self._accum_steps.assign(0)
for gradient in self._gradients:
gradient.assign(tf.zeros_like(gradient))