diffusers/schedulers/scheduling_unclip.py

# Copyright 2025 Kakao Brain and The HuggingFace Team. All rights reserved.
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# Licensed under the Apache License, Version 2.0 (the "License");
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import math
from dataclasses import dataclass
from typing import Literal

import numpy as np
import torch

from ..configuration_utils import ConfigMixin, register_to_config
from ..utils import BaseOutput
from ..utils.torch_utils import randn_tensor
from .scheduling_utils import SchedulerMixin


@dataclass
# Copied from diffusers.schedulers.scheduling_ddpm.DDPMSchedulerOutput with DDPM->UnCLIP
class UnCLIPSchedulerOutput(BaseOutput):
    """
    Output class for the scheduler's `step` function output.

    Args:
        prev_sample (`torch.Tensor` of shape `(batch_size, num_channels, height, width)` for images):
            Computed sample `(x_{t-1})` of previous timestep. `prev_sample` should be used as next model input in the
            denoising loop.
        pred_original_sample (`torch.Tensor` of shape `(batch_size, num_channels, height, width)` for images):
            The predicted denoised sample `(x_{0})` based on the model output from the current timestep.
            `pred_original_sample` can be used to preview progress or for guidance.
    """

    prev_sample: torch.Tensor
    pred_original_sample: torch.Tensor | None = None


# Copied from diffusers.schedulers.scheduling_ddpm.betas_for_alpha_bar
def betas_for_alpha_bar(
    num_diffusion_timesteps: int,
    max_beta: float = 0.999,
    alpha_transform_type: Literal["cosine", "exp", "laplace"] = "cosine",
) -> torch.Tensor:
    """
    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].

    Contains a function alpha_bar that takes an argument t and transforms it to the cumulative product of (1-beta) up
    to that part of the diffusion process.

    Args:
        num_diffusion_timesteps (`int`):
            The number of betas to produce.
        max_beta (`float`, defaults to `0.999`):
            The maximum beta to use; use values lower than 1 to avoid numerical instability.
        alpha_transform_type (`str`, defaults to `"cosine"`):
            The type of noise schedule for `alpha_bar`. Choose from `cosine`, `exp`, or `laplace`.

    Returns:
        `torch.Tensor`:
            The betas used by the scheduler to step the model outputs.
    """
    if alpha_transform_type == "cosine":

        def alpha_bar_fn(t):
            return math.cos((t + 0.008) / 1.008 * math.pi / 2) ** 2

    elif alpha_transform_type == "laplace":

        def alpha_bar_fn(t):
            lmb = -0.5 * math.copysign(1, 0.5 - t) * math.log(1 - 2 * math.fabs(0.5 - t) + 1e-6)
            snr = math.exp(lmb)
            return math.sqrt(snr / (1 + snr))

    elif alpha_transform_type == "exp":

        def alpha_bar_fn(t):
            return math.exp(t * -12.0)

    else:
        raise ValueError(f"Unsupported alpha_transform_type: {alpha_transform_type}")

    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_fn(t2) / alpha_bar_fn(t1), max_beta))
    return torch.tensor(betas, dtype=torch.float32)


class UnCLIPScheduler(SchedulerMixin, ConfigMixin):
    """
    NOTE: do not use this scheduler. The DDPM scheduler has been updated to support the changes made here. This
    scheduler will be removed and replaced with DDPM.

    This is a modified DDPM Scheduler specifically for the karlo unCLIP model.

    This scheduler has some minor variations in how it calculates the learned range variance and dynamically
    re-calculates betas based off the timesteps it is skipping.

    The scheduler also uses a slightly different step ratio when computing timesteps to use for inference.

    See [`~DDPMScheduler`] for more information on DDPM scheduling

    Args:
        num_train_timesteps (`int`, defaults to `1000`):
            The number of diffusion steps used to train the model.
        variance_type (`"fixed_small_log"` or `"learned_range"`, defaults to `"fixed_small_log"`):
            The variance type used when adding noise to the denoised sample.
        clip_sample (`bool`, defaults to `True`):
            Whether to clip the predicted sample between `-clip_sample_range` and `clip_sample_range` for numerical
            stability.
        clip_sample_range (`float`, defaults to `1.0`):
            The range to clip the sample between. See `clip_sample`.
        prediction_type (`"epsilon"` or `"sample"`, defaults to `"epsilon"`):
            The prediction type of the scheduler function. Choose `epsilon` to predict the noise of the diffusion
            process or `sample` to directly predict the noisy sample.
        beta_schedule (`"squaredcos_cap_v2"`, defaults to `"squaredcos_cap_v2"`):
            The beta schedule used to generate the beta values. Only `squaredcos_cap_v2` is supported.
    """

    @register_to_config
    def __init__(
        self,
        num_train_timesteps: int = 1000,
        variance_type: Literal["fixed_small_log", "learned_range"] = "fixed_small_log",
        clip_sample: bool = True,
        clip_sample_range: float = 1.0,
        prediction_type: Literal["epsilon", "sample"] = "epsilon",
        beta_schedule: Literal["squaredcos_cap_v2"] = "squaredcos_cap_v2",
    ) -> None:
        if beta_schedule != "squaredcos_cap_v2":
            raise ValueError("UnCLIPScheduler only supports `beta_schedule`: 'squaredcos_cap_v2'")

        self.betas = betas_for_alpha_bar(num_train_timesteps)

        self.alphas = 1.0 - self.betas
        self.alphas_cumprod = torch.cumprod(self.alphas, dim=0)
        self.one = torch.tensor(1.0)

        # standard deviation of the initial noise distribution
        self.init_noise_sigma = 1.0

        # setable values
        self.num_inference_steps = None
        self.timesteps = torch.from_numpy(np.arange(0, num_train_timesteps)[::-1].copy())

        self.variance_type = variance_type

    def scale_model_input(self, sample: torch.Tensor, timestep: int | None = None) -> torch.Tensor:
        """
        Ensures interchangeability with schedulers that need to scale the denoising model input depending on the
        current timestep.

        Args:
            sample (`torch.Tensor`):
                The input sample.
            timestep (`int`, *optional*):
                The current timestep in the diffusion chain.

        Returns:
            `torch.Tensor`:
                A scaled input sample.
        """
        return sample

    def set_timesteps(self, num_inference_steps: int, device: str | torch.device | None = None) -> None:
        """
        Sets the discrete timesteps used for the diffusion chain. Supporting function to be run before inference.

        Note that this scheduler uses a slightly different step ratio than the other diffusers schedulers. The
        different step ratio is to mimic the original karlo implementation and does not affect the quality or accuracy
        of the results.

        Args:
            num_inference_steps (`int`):
                The number of diffusion steps used when generating samples with a pre-trained model.
            device (`str` or `torch.device`, *optional*):
                The device to which the timesteps are moved. If `None`, the timesteps are not moved.
        """
        self.num_inference_steps = num_inference_steps
        step_ratio = (self.config.num_train_timesteps - 1) / (self.num_inference_steps - 1)
        timesteps = (np.arange(0, num_inference_steps) * step_ratio).round()[::-1].copy().astype(np.int64)
        self.timesteps = torch.from_numpy(timesteps).to(device)

    def _get_variance(
        self,
        t: int,
        prev_timestep: int | None = None,
        predicted_variance: torch.Tensor | None = None,
        variance_type: Literal["fixed_small_log", "learned_range"] | None = None,
    ) -> torch.Tensor:
        if prev_timestep is None:
            prev_timestep = t - 1

        alpha_prod_t = self.alphas_cumprod[t]
        alpha_prod_t_prev = self.alphas_cumprod[prev_timestep] if prev_timestep >= 0 else self.one
        beta_prod_t = 1 - alpha_prod_t
        beta_prod_t_prev = 1 - alpha_prod_t_prev

        if prev_timestep == t - 1:
            beta = self.betas[t]
        else:
            beta = 1 - alpha_prod_t / alpha_prod_t_prev

        # For t > 0, compute predicted variance βt (see formula (6) and (7) from https://huggingface.co/papers/2006.11239)
        # and sample from it to get previous sample
        # x_{t-1} ~ N(pred_prev_sample, variance) == add variance to pred_sample
        variance = beta_prod_t_prev / beta_prod_t * beta

        if variance_type is None:
            variance_type = self.config.variance_type

        # hacks - were probably added for training stability
        if variance_type == "fixed_small_log":
            variance = torch.log(torch.clamp(variance, min=1e-20))
            variance = torch.exp(0.5 * variance)
        elif variance_type == "learned_range":
            # NOTE difference with DDPM scheduler
            min_log = variance.log()
            max_log = beta.log()

            frac = (predicted_variance + 1) / 2
            variance = frac * max_log + (1 - frac) * min_log

        return variance

    def step(
        self,
        model_output: torch.Tensor,
        timestep: int,
        sample: torch.Tensor,
        prev_timestep: int | None = None,
        generator: torch.Generator | None = None,
        return_dict: bool = True,
    ) -> UnCLIPSchedulerOutput | tuple:
        """
        Predict the sample at the previous timestep by reversing the SDE. Core function to propagate the diffusion
        process from the learned model outputs (most often the predicted noise).

        Args:
            model_output (`torch.Tensor`):
                The direct output from the learned diffusion model.
            timestep (`int`):
                The current discrete timestep in the diffusion chain.
            sample (`torch.Tensor`):
                A current instance of a sample created by the diffusion process.
            prev_timestep (`int`, *optional*):
                The previous timestep for which to predict the previous sample. It is used to dynamically compute beta.
                If `None`, `t-1` and the precomputed beta are used.
            generator (`torch.Generator`, *optional*):
                A random number generator.
            return_dict (`bool`, defaults to `True`):
                Whether to return an [`~schedulers.scheduling_unclip.UnCLIPSchedulerOutput`] or a `tuple`.

        Returns:
            [`~schedulers.scheduling_unclip.UnCLIPSchedulerOutput`] or `tuple`:
                If `return_dict` is `True`, an [`~schedulers.scheduling_unclip.UnCLIPSchedulerOutput`] is returned;
                otherwise, a tuple is returned where the first element is the sample tensor.
        """
        t = timestep

        if model_output.shape[1] == sample.shape[1] * 2 and self.variance_type == "learned_range":
            model_output, predicted_variance = torch.split(model_output, sample.shape[1], dim=1)
        else:
            predicted_variance = None

        # 1. compute alphas, betas
        if prev_timestep is None:
            prev_timestep = t - 1

        alpha_prod_t = self.alphas_cumprod[t]
        alpha_prod_t_prev = self.alphas_cumprod[prev_timestep] if prev_timestep >= 0 else self.one
        beta_prod_t = 1 - alpha_prod_t
        beta_prod_t_prev = 1 - alpha_prod_t_prev

        if prev_timestep == t - 1:
            beta = self.betas[t]
            alpha = self.alphas[t]
        else:
            beta = 1 - alpha_prod_t / alpha_prod_t_prev
            alpha = 1 - beta

        # 2. compute predicted original sample from predicted noise also called
        # "predicted x_0" of formula (15) from https://huggingface.co/papers/2006.11239
        if self.config.prediction_type == "epsilon":
            pred_original_sample = (sample - beta_prod_t ** (0.5) * model_output) / alpha_prod_t ** (0.5)
        elif self.config.prediction_type == "sample":
            pred_original_sample = model_output
        else:
            raise ValueError(
                f"prediction_type given as {self.config.prediction_type} must be one of `epsilon` or `sample`"
                " for the UnCLIPScheduler."
            )

        # 3. Clip "predicted x_0"
        if self.config.clip_sample:
            pred_original_sample = torch.clamp(
                pred_original_sample, -self.config.clip_sample_range, self.config.clip_sample_range
            )

        # 4. Compute coefficients for pred_original_sample x_0 and current sample x_t
        # See formula (7) from https://huggingface.co/papers/2006.11239
        pred_original_sample_coeff = (alpha_prod_t_prev ** (0.5) * beta) / beta_prod_t
        current_sample_coeff = alpha ** (0.5) * beta_prod_t_prev / beta_prod_t

        # 5. Compute predicted previous sample µ_t
        # See formula (7) from https://huggingface.co/papers/2006.11239
        pred_prev_sample = pred_original_sample_coeff * pred_original_sample + current_sample_coeff * sample

        # 6. Add noise
        variance = 0
        if t > 0:
            variance_noise = randn_tensor(
                model_output.shape, dtype=model_output.dtype, generator=generator, device=model_output.device
            )

            variance = self._get_variance(
                t,
                predicted_variance=predicted_variance,
                prev_timestep=prev_timestep,
            )

            if self.variance_type == "fixed_small_log":
                variance = variance
            elif self.variance_type == "learned_range":
                variance = (0.5 * variance).exp()
            else:
                raise ValueError(
                    f"variance_type given as {self.variance_type} must be one of `fixed_small_log` or `learned_range`"
                    " for the UnCLIPScheduler."
                )

            variance = variance * variance_noise

        pred_prev_sample = pred_prev_sample + variance

        if not return_dict:
            return (
                pred_prev_sample,
                pred_original_sample,
            )

        return UnCLIPSchedulerOutput(prev_sample=pred_prev_sample, pred_original_sample=pred_original_sample)

    # Copied from diffusers.schedulers.scheduling_ddpm.DDPMScheduler.add_noise
    def add_noise(
        self,
        original_samples: torch.Tensor,
        noise: torch.Tensor,
        timesteps: torch.IntTensor,
    ) -> torch.Tensor:
        """
        Add noise to the original samples according to the noise magnitude at each timestep (this is the forward
        diffusion process).

        Args:
            original_samples (`torch.Tensor`):
                The original samples to which noise will be added.
            noise (`torch.Tensor`):
                The noise to add to the samples.
            timesteps (`torch.IntTensor`):
                The timesteps indicating the noise level for each sample.

        Returns:
            `torch.Tensor`:
                The noisy samples.
        """
        # Make sure alphas_cumprod and timestep have same device and dtype as original_samples
        # Move the self.alphas_cumprod to device to avoid redundant CPU to GPU data movement
        # for the subsequent add_noise calls
        self.alphas_cumprod = self.alphas_cumprod.to(device=original_samples.device)
        alphas_cumprod = self.alphas_cumprod.to(dtype=original_samples.dtype)
        timesteps = timesteps.to(original_samples.device)

        sqrt_alpha_prod = alphas_cumprod[timesteps] ** 0.5
        sqrt_alpha_prod = sqrt_alpha_prod.flatten()
        while len(sqrt_alpha_prod.shape) < len(original_samples.shape):
            sqrt_alpha_prod = sqrt_alpha_prod.unsqueeze(-1)

        sqrt_one_minus_alpha_prod = (1 - alphas_cumprod[timesteps]) ** 0.5
        sqrt_one_minus_alpha_prod = sqrt_one_minus_alpha_prod.flatten()
        while len(sqrt_one_minus_alpha_prod.shape) < len(original_samples.shape):
            sqrt_one_minus_alpha_prod = sqrt_one_minus_alpha_prod.unsqueeze(-1)

        noisy_samples = sqrt_alpha_prod * original_samples + sqrt_one_minus_alpha_prod * noise
        return noisy_samples