gaussian_ddpm.py

# Copyright 2022 The HuggingFace Team. 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.
import torch
import math
from torch import nn

from ..configuration_utils import ConfigMixin


SAMPLING_CONFIG_NAME = "scheduler_config.json"


def linear_beta_schedule(timesteps, beta_start, beta_end):
    return torch.linspace(beta_start, beta_end, timesteps, dtype=torch.float64)


def betas_for_alpha_bar(num_diffusion_timesteps, alpha_bar, max_beta=0.999):
    """
    Create a beta schedule that discretizes the given alpha_t_bar function,
    which defines the cumulative product of (1-beta) over time from t = [0,1].

    :param num_diffusion_timesteps: the number of betas to produce.
    :param alpha_bar: a lambda that takes an argument t from 0 to 1 and
                      produces the cumulative product of (1-beta) up to that
                      part of the diffusion process.
    :param max_beta: the maximum beta to use; use values lower than 1 to
                     prevent singularities.
    """
    betas = []
    for i in range(num_diffusion_timesteps):
        t1 = i / num_diffusion_timesteps
        t2 = (i + 1) / num_diffusion_timesteps
        betas.append(min(1 - alpha_bar(t2) / alpha_bar(t1), max_beta))
    return torch.tensor(betas, dtype=torch.float64)


class GaussianDDPMScheduler(nn.Module, ConfigMixin):

    config_name = SAMPLING_CONFIG_NAME

    def __init__(
        self,
        timesteps=1000,
        beta_start=0.0001,
        beta_end=0.02,
        beta_schedule="linear",
        variance_type="fixed_small",
    ):
        super().__init__()
        self.register(
            timesteps=timesteps,
            beta_start=beta_start,
            beta_end=beta_end,
            beta_schedule=beta_schedule,
            variance_type=variance_type,
        )
        self.num_timesteps = int(timesteps)

        if beta_schedule == "linear":
            betas = linear_beta_schedule(timesteps, beta_start=beta_start, beta_end=beta_end)
        elif beta_schedule == "squaredcos_cap_v2":
            # GLIDE cosine schedule
            betas = betas_for_alpha_bar(
                timesteps,
                lambda t: math.cos((t + 0.008) / 1.008 * math.pi / 2) ** 2,
            )
        else:
            raise NotImplementedError(f"{beta_schedule} does is not implemented for {self.__class__}")

        alphas = 1.0 - betas
        alphas_cumprod = torch.cumprod(alphas, axis=0)
        alphas_cumprod_prev = torch.nn.functional.pad(alphas_cumprod[:-1], (1, 0), value=1.0)

        variance = betas * (1.0 - alphas_cumprod_prev) / (1.0 - alphas_cumprod)

        if variance_type == "fixed_small":
            log_variance = torch.log(variance.clamp(min=1e-20))
        elif variance_type == "fixed_large":
            log_variance = torch.log(torch.cat([variance[1:2], betas[1:]], dim=0))

        self.register_buffer("betas", betas.to(torch.float32))
        self.register_buffer("alphas", alphas.to(torch.float32))
        self.register_buffer("alphas_cumprod", alphas_cumprod.to(torch.float32))

        self.register_buffer("log_variance", log_variance.to(torch.float32))

    def get_alpha(self, time_step):
        return self.alphas[time_step]

    def get_beta(self, time_step):
        return self.betas[time_step]

    def get_alpha_prod(self, time_step):
        if time_step < 0:
            return torch.tensor(1.0)
        return self.alphas_cumprod[time_step]

    def sample_variance(self, time_step, shape, device, generator=None):
        variance = self.log_variance[time_step]
        nonzero_mask = torch.tensor([1 - (time_step == 0)], device=device).float()[None, :]

        noise = self.sample_noise(shape, device=device, generator=generator)

        sampled_variance = nonzero_mask * (0.5 * variance).exp()
        sampled_variance = sampled_variance * noise

        return sampled_variance

    def sample_noise(self, shape, device, generator=None):
        # always sample on CPU to be deterministic
        return torch.randn(shape, generator=generator).to(device)

    def __len__(self):
        return self.num_timesteps