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# Copyright (c) 2020 Johns Hopkins University (Shinji Watanabe)
# 2020 Northwestern Polytechnical University (Pengcheng Guo)
# 2020 Mobvoi Inc (Binbin Zhang)
# 2024 Alibaba Inc (Xiang Lyu)
#
# 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.
"""Swish() activation function for Conformer."""
import torch
from torch import nn, sin, pow
from torch.nn import Parameter
class Swish(torch.nn.Module):
"""Construct an Swish object."""
def forward(self, x: torch.Tensor) -> torch.Tensor:
"""Return Swish activation function."""
return x * torch.sigmoid(x)
# Implementation adapted from https://github.com/EdwardDixon/snake under the MIT license.
# LICENSE is in incl_licenses directory.
class Snake(nn.Module):
'''
Implementation of a sine-based periodic activation function
Shape:
- Input: (B, C, T)
- Output: (B, C, T), same shape as the input
Parameters:
- alpha - trainable parameter
References:
- This activation function is from this paper by Liu Ziyin, Tilman Hartwig, Masahito Ueda:
https://arxiv.org/abs/2006.08195
Examples:
>>> a1 = snake(256)
>>> x = torch.randn(256)
>>> x = a1(x)
'''
def __init__(self, in_features, alpha=1.0, alpha_trainable=True, alpha_logscale=False):
'''
Initialization.
INPUT:
- in_features: shape of the input
- alpha: trainable parameter
alpha is initialized to 1 by default, higher values = higher-frequency.
alpha will be trained along with the rest of your model.
'''
super(Snake, self).__init__()
self.in_features = in_features
# initialize alpha
self.alpha_logscale = alpha_logscale
if self.alpha_logscale: # log scale alphas initialized to zeros
self.alpha = Parameter(torch.zeros(in_features) * alpha)
else: # linear scale alphas initialized to ones
self.alpha = Parameter(torch.ones(in_features) * alpha)
self.alpha.requires_grad = alpha_trainable
self.no_div_by_zero = 0.000000001
def forward(self, x):
'''
Forward pass of the function.
Applies the function to the input elementwise.
Snake ∶= x + 1/a * sin^2 (xa)
'''
alpha = self.alpha.unsqueeze(0).unsqueeze(-1) # line up with x to [B, C, T]
if self.alpha_logscale:
alpha = torch.exp(alpha)
x = x + (1.0 / (alpha + self.no_div_by_zero)) * pow(sin(x * alpha), 2)
return x
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