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# This source code is licensed under the license found in the LICENSE file in
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# can be found in the PATENTS file in the same directory.
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#-------------------------------------------------------------------------
#
# Copyright (c) 2019, NVIDIA CORPORATION. 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
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#     http://www.apache.org/licenses/LICENSE-2.0
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import torch.optim


class FairseqOptimizer(object):

    def __init__(self, args, params):
        super().__init__()
        self.args = args
        self.params = params

    @staticmethod
    def add_args(parser):
        """Add optimizer-specific arguments to the parser."""
        pass

    @property
    def optimizer(self):
        """Return a torch.optim.optimizer.Optimizer instance."""
        if not hasattr(self, '_optimizer'):
            raise NotImplementedError
        if not isinstance(self._optimizer, torch.optim.Optimizer):
            raise ValueError('_optimizer must be an instance of torch.optim.Optimizer')
        return self._optimizer

    @property
    def optimizer_config(self):
        """
        Return a kwarg dictionary that will be used to override optimizer
        args stored in checkpoints. This allows us to load a checkpoint and
        resume training using a different set of optimizer args, e.g., with a
        different learning rate.
        """
        raise NotImplementedError

    def get_lr(self):
        """Return the current learning rate."""
        return self.optimizer.param_groups[0]['lr']

    def set_lr(self, lr):
        """Set the learning rate."""
        for param_group in self.optimizer.param_groups:
            param_group['lr'] = lr

    def state_dict(self):
        """Return the optimizer's state dict."""
        return self.optimizer.state_dict()

    def load_state_dict(self, state_dict):
        """Load an optimizer state dict.

        In general we should prefer the configuration of the existing optimizer
        instance (e.g., learning rate) over that found in the state_dict. This
        allows us to resume training from a checkpoint using a new set of
        optimizer args.
        """
        self.optimizer.load_state_dict(state_dict)

        # override learning rate, momentum, etc. with latest values
        for group in self.optimizer.param_groups:
            group.update(self.optimizer_config)

    def step(self, closure=None):
        """Performs a single optimization step."""
        return self.optimizer.step(closure)

    def zero_grad(self):
        """Clears the gradients of all optimized parameters."""
        for group in self.optimizer.param_groups:
            for p in group['params']:
                p.grad = None

        return self.optimizer.zero_grad()
