noise_schedule_noise_schedules.py

from abc import ABC, abstractmethod

import torch


class Noise(ABC):
    """
    Baseline forward method to get noise parameters at a timestep
    """

    def __call__(
        self, t: torch.Tensor | float
    ) -> tuple[torch.Tensor | float, torch.Tensor | float]:
        # Assume time goes from 0 to 1
        pass

    @abstractmethod
    def inverse(self, alpha_t: torch.Tensor) -> torch.Tensor:
        """
        Inverse function to compute the timestep t from the noise schedule param.
        """
        raise NotImplementedError("Inverse function not implemented")


class CosineNoise(Noise):
    def __init__(self, eps=1e-3):
        super().__init__()
        self.eps = eps
        self.name = "cosine"

    def __call__(self, t):
        t = t.to(torch.float32)
        cos = -(1 - self.eps) * torch.cos(t * torch.pi / 2)
        sin = -(1 - self.eps) * torch.sin(t * torch.pi / 2)
        move_chance = cos + 1
        alpha_t_prime = sin * torch.pi / 2
        return 1 - move_chance, alpha_t_prime


class ExponentialNoise(Noise):
    def __init__(self, exp=2, eps=1e-3):
        super().__init__()
        self.eps = eps
        self.exp = exp
        self.name = f"exp_{exp}"

    def __call__(self, t):
        t = t.to(torch.float32)
        move_chance = torch.pow(t, self.exp)
        move_chance = torch.clamp(move_chance, min=self.eps)
        alpha_t_prime = -self.exp * torch.pow(t, self.exp - 1)
        return alpha_t_prime, 1 - move_chance


class LogarithmicNoise(Noise):
    def __init__(self, eps=1e-3):
        super().__init__()
        self.eps = eps
        self.name = "logarithmic"

    def __call__(self, t):
        t = t.to(torch.float32)
        move_chance = torch.log1p(t) / torch.log(torch.tensor(2.0))
        alpha_t_prime = -1 / (torch.log(torch.tensor(2.0)) * (1 + t))
        return 1 - move_chance, alpha_t_prime


class LinearNoise(Noise):
    def __init__(self):
        super().__init__()
        self.name = "linear"

    def inverse(self, alpha_t):
        return 1 - alpha_t

    def __call__(self, t):
        t = t.to(torch.float32)
        alpha_t_prime = -torch.ones_like(t)
        move_chance = t
        return 1 - move_chance, alpha_t_prime