|
import math |
|
from dataclasses import dataclass |
|
from typing import Optional, Tuple, Union |
|
|
|
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 |
|
|
|
|
|
|
|
def betas_for_alpha_bar( |
|
num_diffusion_timesteps, |
|
max_beta=0.999, |
|
alpha_transform_type="cosine", |
|
): |
|
""" |
|
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`): the maximum beta to use; use values lower than 1 to |
|
prevent singularities. |
|
alpha_transform_type (`str`, *optional*, default to `cosine`): the type of noise schedule for alpha_bar. |
|
Choose from `cosine` or `exp` |
|
|
|
Returns: |
|
betas (`np.ndarray`): 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 == "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) |
|
|
|
|
|
@dataclass |
|
class ConsistencyDecoderSchedulerOutput(BaseOutput): |
|
""" |
|
Output class for the scheduler's `step` function. |
|
|
|
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. |
|
""" |
|
|
|
prev_sample: torch.Tensor |
|
|
|
|
|
class ConsistencyDecoderScheduler(SchedulerMixin, ConfigMixin): |
|
order = 1 |
|
|
|
@register_to_config |
|
def __init__( |
|
self, |
|
num_train_timesteps: int = 1024, |
|
sigma_data: float = 0.5, |
|
): |
|
betas = betas_for_alpha_bar(num_train_timesteps) |
|
|
|
alphas = 1.0 - betas |
|
alphas_cumprod = torch.cumprod(alphas, dim=0) |
|
|
|
self.sqrt_alphas_cumprod = torch.sqrt(alphas_cumprod) |
|
self.sqrt_one_minus_alphas_cumprod = torch.sqrt(1.0 - alphas_cumprod) |
|
|
|
sigmas = torch.sqrt(1.0 / alphas_cumprod - 1) |
|
|
|
sqrt_recip_alphas_cumprod = torch.sqrt(1.0 / alphas_cumprod) |
|
|
|
self.c_skip = sqrt_recip_alphas_cumprod * sigma_data**2 / (sigmas**2 + sigma_data**2) |
|
self.c_out = sigmas * sigma_data / (sigmas**2 + sigma_data**2) ** 0.5 |
|
self.c_in = sqrt_recip_alphas_cumprod / (sigmas**2 + sigma_data**2) ** 0.5 |
|
|
|
def set_timesteps( |
|
self, |
|
num_inference_steps: Optional[int] = None, |
|
device: Union[str, torch.device] = None, |
|
): |
|
if num_inference_steps != 2: |
|
raise ValueError("Currently more than 2 inference steps are not supported.") |
|
|
|
self.timesteps = torch.tensor([1008, 512], dtype=torch.long, device=device) |
|
self.sqrt_alphas_cumprod = self.sqrt_alphas_cumprod.to(device) |
|
self.sqrt_one_minus_alphas_cumprod = self.sqrt_one_minus_alphas_cumprod.to(device) |
|
self.c_skip = self.c_skip.to(device) |
|
self.c_out = self.c_out.to(device) |
|
self.c_in = self.c_in.to(device) |
|
|
|
@property |
|
def init_noise_sigma(self): |
|
return self.sqrt_one_minus_alphas_cumprod[self.timesteps[0]] |
|
|
|
def scale_model_input(self, sample: torch.Tensor, timestep: Optional[int] = 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 * self.c_in[timestep] |
|
|
|
def step( |
|
self, |
|
model_output: torch.Tensor, |
|
timestep: Union[float, torch.Tensor], |
|
sample: torch.Tensor, |
|
generator: Optional[torch.Generator] = None, |
|
return_dict: bool = True, |
|
) -> Union[ConsistencyDecoderSchedulerOutput, Tuple]: |
|
""" |
|
Predict the sample from the previous timestep by reversing the SDE. This function propagates 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 (`float`): |
|
The current timestep in the diffusion chain. |
|
sample (`torch.Tensor`): |
|
A current instance of a sample created by the diffusion process. |
|
generator (`torch.Generator`, *optional*): |
|
A random number generator. |
|
return_dict (`bool`, *optional*, defaults to `True`): |
|
Whether or not to return a |
|
[`~schedulers.scheduling_consistency_models.ConsistencyDecoderSchedulerOutput`] or `tuple`. |
|
|
|
Returns: |
|
[`~schedulers.scheduling_consistency_models.ConsistencyDecoderSchedulerOutput`] or `tuple`: |
|
If return_dict is `True`, |
|
[`~schedulers.scheduling_consistency_models.ConsistencyDecoderSchedulerOutput`] is returned, otherwise |
|
a tuple is returned where the first element is the sample tensor. |
|
""" |
|
x_0 = self.c_out[timestep] * model_output + self.c_skip[timestep] * sample |
|
|
|
timestep_idx = torch.where(self.timesteps == timestep)[0] |
|
|
|
if timestep_idx == len(self.timesteps) - 1: |
|
prev_sample = x_0 |
|
else: |
|
noise = randn_tensor(x_0.shape, generator=generator, dtype=x_0.dtype, device=x_0.device) |
|
prev_sample = ( |
|
self.sqrt_alphas_cumprod[self.timesteps[timestep_idx + 1]].to(x_0.dtype) * x_0 |
|
+ self.sqrt_one_minus_alphas_cumprod[self.timesteps[timestep_idx + 1]].to(x_0.dtype) * noise |
|
) |
|
|
|
if not return_dict: |
|
return (prev_sample,) |
|
|
|
return ConsistencyDecoderSchedulerOutput(prev_sample=prev_sample) |
|
|
