""" Lion Optimizer
Paper: `Symbolic Discovery of Optimization Algorithms` - https://arxiv.org/abs/2302.06675
Original Impl: https://github.com/google/automl/tree/master/lion
"""
# Copyright 2023 Google Research. 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.
# ==============================================================================
from typing import List
import torch
from torch.optim.optimizer import Optimizer
class Lion(Optimizer):
r"""Implements Lion algorithm."""
def __init__(
self,
params,
lr=1e-4,
betas=(0.9, 0.99),
weight_decay=0.0,
maximize=False,
foreach=None,
):
"""Initialize the hyperparameters.
Args:
params (iterable): iterable of parameters to optimize or dicts defining
parameter groups
lr (float, optional): learning rate (default: 1e-4)
betas (Tuple[float, float], optional): coefficients used for computing
running averages of gradient and its square (default: (0.9, 0.99))
weight_decay (float, optional): weight decay coefficient (default: 0)
"""
if not 0.0 <= lr:
raise ValueError('Invalid learning rate: {}'.format(lr))
if not 0.0 <= betas[0] < 1.0:
raise ValueError('Invalid beta parameter at index 0: {}'.format(betas[0]))
if not 0.0 <= betas[1] < 1.0:
raise ValueError('Invalid beta parameter at index 1: {}'.format(betas[1]))
defaults = dict(
lr=lr,
betas=betas,
weight_decay=weight_decay,
foreach=foreach,
maximize=maximize,
)
super().__init__(params, defaults)
def __setstate__(self, state):
super().__setstate__(state)
for group in self.param_groups:
group.setdefault('maximize', False)
group.setdefault('foreach', None)
@torch.no_grad()
def step(self, closure=None):
"""Performs a single optimization step.
Args:
closure (callable, optional): A closure that reevaluates the model
and returns the loss.
Returns:
the loss.
"""
loss = None
if closure is not None:
with torch.enable_grad():
loss = closure()
for group in self.param_groups:
params_with_grad = []
grads = []
exp_avgs = []
beta1, beta2 = group['betas']
for p in group['params']:
if p.grad is None:
continue
params_with_grad.append(p)
if p.grad.is_sparse:
raise RuntimeError('Lion does not support sparse gradients')
grads.append(p.grad)
state = self.state[p]
# State initialization
if len(state) == 0:
state['exp_avg'] = torch.zeros_like(p, memory_format=torch.preserve_format)
exp_avgs.append(state['exp_avg'])
lion(
params_with_grad,
grads,
exp_avgs,
beta1=beta1,
beta2=beta2,
lr=group['lr'],
weight_decay=group['weight_decay'],
maximize=group['maximize'],
foreach=group['foreach'],
)
return loss
def lion(
params: List[torch.Tensor],
grads: List[torch.Tensor],
exp_avgs: List[torch.Tensor],
# kwonly args with defaults are not supported by functions compiled with torchscript issue #70627
# setting this as kwarg for now as functional API is compiled by torch/distributed/optim
maximize: bool = False,
foreach: bool = None,
*,
beta1: float,
beta2: float,
lr: float,
weight_decay: float,
):
r"""Functional API that performs Lion algorithm computation.
"""
if foreach is None:
# Placeholder for more complex foreach logic to be added when value is not set
foreach = False
if foreach and torch.jit.is_scripting():
raise RuntimeError('torch.jit.script not supported with foreach optimizers')
if foreach and not torch.jit.is_scripting():
func = _multi_tensor_lion
else:
func = _single_tensor_lion
func(
params,
grads,
exp_avgs,
beta1=beta1,
beta2=beta2,
lr=lr,
weight_decay=weight_decay,
maximize=maximize,
)
def _single_tensor_lion(
params: List[torch.Tensor],
grads: List[torch.Tensor],
exp_avgs: List[torch.Tensor],
*,
beta1: float,
beta2: float,
lr: float,
weight_decay: float,
maximize: bool,
):
for i, param in enumerate(params):
grad = grads[i] if not maximize else -grads[i]
exp_avg = exp_avgs[i]
if torch.is_complex(param):
grad = torch.view_as_real(grad)
exp_avg = torch.view_as_real(exp_avg)
param = torch.view_as_real(param)
# Perform stepweight decay
param.mul_(1 - lr * weight_decay)
# Weight update
update = exp_avg.mul(beta1).add_(grad, alpha=1 - beta1)
param.add_(torch.sign(update), alpha=-lr)
# Decay the momentum running average coefficient
exp_avg.lerp_(grad, 1 - beta2)
def _multi_tensor_lion(
params: List[torch.Tensor],
grads: List[torch.Tensor],
exp_avgs: List[torch.Tensor],
*,
beta1: float,
beta2: float,
lr: float,
weight_decay: float,
maximize: bool,
):
if len(params) == 0:
return
if maximize:
grads = torch._foreach_neg(tuple(grads)) # type: ignore[assignment]
grads = [torch.view_as_real(x) if torch.is_complex(x) else x for x in grads]
exp_avgs = [torch.view_as_real(x) if torch.is_complex(x) else x for x in exp_avgs]
params = [torch.view_as_real(x) if torch.is_complex(x) else x for x in params]
# Perform stepweight decay
torch._foreach_mul_(params, 1 - lr * weight_decay)
# Weight update
updates = torch._foreach_mul(exp_avgs, beta1)
torch._foreach_add_(updates, grads, alpha=1 - beta1)
updates = [u.sign() for u in updates]
torch._foreach_add_(params, updates, alpha=-lr)
# Decay the momentum running average coefficient
torch._foreach_mul_(exp_avgs, beta2)
torch._foreach_add_(exp_avgs, grads, alpha=1 - beta2)