add some examples to seperate sampler and schedules

examples/sample_loop.py

+#!/usr/bin/env python3
+from diffusers import UNetModel, GaussianDiffusion
+import torch
+import torch.nn.functional as F
+
+unet = UNetModel.from_pretrained("fusing/ddpm_dummy")
+diffusion = GaussianDiffusion.from_config("fusing/ddpm_dummy")
+
+# 2. Do one denoising step with model
+batch_size, num_channels, height, width = 1, 3, 32, 32
+dummy_noise = torch.ones((batch_size, num_channels, height, width))
+
+
+TIME_STEPS = 10
+
+
+# Helper
+def extract(a, t, x_shape):
+    b, *_ = t.shape
+    out = a.gather(-1, t)
+    return out.reshape(b, *((1,) * (len(x_shape) - 1)))
+
+
+def noise_like(shape, device, repeat=False):
+    def repeat_noise():
+        return torch.randn((1, *shape[1:]), device=device).repeat(shape[0], *((1,) * (len(shape) - 1)))
+
+    def noise():
+        return torch.randn(shape, device=device)
+
+    return repeat_noise() if repeat else noise()
+
+
+# Schedule
+def cosine_beta_schedule(timesteps, s=0.008):
+    """
+    cosine schedule
+    as proposed in https://openreview.net/forum?id=-NEXDKk8gZ
+    """
+    steps = timesteps + 1
+    x = torch.linspace(0, timesteps, steps, dtype=torch.float64)
+    alphas_cumprod = torch.cos(((x / timesteps) + s) / (1 + s) * torch.pi * 0.5) ** 2
+    alphas_cumprod = alphas_cumprod / alphas_cumprod[0]
+    betas = 1 - (alphas_cumprod[1:] / alphas_cumprod[:-1])
+    return torch.clip(betas, 0, 0.999)
+
+
+betas = cosine_beta_schedule(TIME_STEPS)
+alphas = 1.0 - betas
+alphas_cumprod = torch.cumprod(alphas, axis=0)
+alphas_cumprod_prev = F.pad(alphas_cumprod[:-1], (1, 0), value=1.0)
+
+posterior_mean_coef1 = betas * torch.sqrt(alphas_cumprod_prev) / (1.0 - alphas_cumprod)
+posterior_mean_coef2 = (1.0 - alphas_cumprod_prev) * torch.sqrt(alphas) / (1.0 - alphas_cumprod)
+
+posterior_variance = betas * (1.0 - alphas_cumprod_prev) / (1.0 - alphas_cumprod)
+posterior_log_variance_clipped = torch.log(posterior_variance.clamp(min=1e-20))
+
+
+sqrt_recip_alphas_cumprod = torch.sqrt(1.0 / alphas_cumprod)
+sqrt_recipm1_alphas_cumprod = torch.sqrt(1.0 / alphas_cumprod - 1)
+
+
+x_t = dummy_noise
+for i in reversed(range(TIME_STEPS)):
+    # t for x_t
+    t = torch.tensor([i])
+    torch.manual_seed(0)
+    noise = noise_like(x_t.shape, "cpu")
+
+    x_t2 = diffusion.p_sample(unet, x_t, t, noise=noise)
+
+    # ------------------------- MODEL ------------------------------------#
+    # predict epsilon
+    pred_noise = unet(x_t, t)
+    pred_x = extract(sqrt_recip_alphas_cumprod, t, x_t.shape) * x_t - extract(sqrt_recipm1_alphas_cumprod, t, x_t.shape) * pred_noise
+    pred_x.clamp_(-1.0, 1.0)
+    posterior_mean = extract(posterior_mean_coef1, t, x_t.shape) * pred_x + extract(posterior_mean_coef2, t, x_t.shape) * x_t
+    # --------------------------------------------------------------------#
+
+    # predict x_{t-1} (=pred_x)
+
+    # ------------------------- Variance Scheduler -----------------------#
+    posterior_log_variance = extract(posterior_log_variance_clipped, t, x_t.shape)
+    # no noise when t == 0
+    b, *_, device = *x_t.shape, x_t.device
+    nonzero_mask = (1 - (t == 0).float()).reshape(b, *((1,) * (len(x_t.shape) - 1)))
+    posterior_variance = nonzero_mask * (0.5 * posterior_log_variance).exp()
+    # --------------------------------------------------------------------#
+
+    x_t = posterior_mean + posterior_variance * noise
+    x_t = x_t.to(torch.float32)
+
+    # make sure manual loop is equal to function
+    assert (x_t - x_t2).abs().sum().item() < 1e-3

src/diffusers/samplers/gaussian.py

@@ -219,10 +219,11 @@ class GaussianDiffusion(nn.Module, Config):
         return model_mean, posterior_variance, posterior_log_variance
 
     @torch.no_grad()
-    def p_sample(self, model, x, t, clip_denoised=True, repeat_noise=False):
+    def p_sample(self, model, x, t, noise=None, clip_denoised=True, repeat_noise=False):
         b, *_, device = *x.shape, x.device
         model_mean, _, model_log_variance = self.p_mean_variance(model=model, x=x, t=t, clip_denoised=clip_denoised)
-        noise = noise_like(x.shape, device, repeat_noise)
+        if noise is None:
+            noise = noise_like(x.shape, device, repeat_noise)
         # no noise when t == 0
         nonzero_mask = (1 - (t == 0).float()).reshape(b, *((1,) * (len(x.shape) - 1)))
         result = model_mean + nonzero_mask * (0.5 * model_log_variance).exp() * noise