megengine.optimizer.sgd 源代码

# -*- coding: utf-8 -*-
import os
from typing import Iterable, Union

from ..core import _config
from ..functional.inplace import _inplace_add_
from ..tensor import Parameter, tensor
from .optimizer import Optimizer

[文档]class SGD(Optimizer): r"""Implements stochastic gradient descent. This optimizer performs stochastic gradient descent with optional momentum and weight decay. Nesterov momentum is based on the formula from `"On the importance of initialization and momentum in deep learning" <>`_. Args: params (Union[Iterable[Parameter], dict]): Iterable of parameters to optimize or dicts defining parameter groups. lr (float): Learning rate. momentum (float): Momentum factor. Default: 0.0. nesterov (bool): Enables Nesterov momentum. Default: False. weight_decay (float): Weight decay (L2 penalty). Default: 0.0. Returns: An instance of the SGD optimizer. Note: This optimizer does not guarantee that the interval does not include the stop value in cases where the step is not an integer and floating-point rounding errors affect the length of the output tensor. """ def __init__( self, params: Union[Iterable[Parameter], dict], lr: float, momentum: float = 0.0, nesterov: bool = False, weight_decay: float = 0.0, ): assert lr >= 0.0, "Invalid learning rate: {}".format(lr) assert momentum >= 0.0, "Invalid momentum value: {}".format(momentum) assert weight_decay >= 0.0, "Invalid weight_decay value: {}".format( weight_decay ) assert not nesterov or momentum > 0.0, "Nesterov momentum requires a momentum" defaults = dict(lr=lr, momentum=momentum, weight_decay=weight_decay) super().__init__(params, defaults) self.nesterov = nesterov self._disable_type_convert = True def _create_state(self, param_group): if param_group["momentum"] != 0.0: for param in param_group["params"]: self._add_state(param, "momentum_buffer") def _updates(self, param_group): lr = param_group["lr"] weight_decay = param_group["weight_decay"] momentum = param_group["momentum"] # since `conver_inputs` is disabled for param updates, # scalar should be explicitly tansforred to tensor _lr = tensor(lr, dtype="float32") _weight_decay = tensor(weight_decay, dtype="float32") _momentum = tensor(momentum, dtype="float32") inplace_mode = int(os.getenv("MEGENGINE_INPLACE_UPDATE", "0")) if inplace_mode: _neg_lr = tensor(-lr, dtype="float32") c1 = tensor(1.0) for param in param_group["params"]: if param.grad is None: continue grad = param.grad if weight_decay != 0.0: grad = grad + param * _weight_decay if inplace_mode: if momentum != 0.0: v = self._state[param]["momentum_buffer"] _inplace_add_(v, grad, alpha=_momentum, beta=c1) if self.nesterov: grad = grad + v * _momentum else: grad = v _inplace_add_(param, grad, alpha=c1, beta=_neg_lr) continue if momentum != 0.0: v = self._state[param]["momentum_buffer"] v *= _momentum v += grad if self.nesterov: grad = grad + v * _momentum else: grad = v param -= _lr * grad