make sure learned variance can work for any number of unets in the decoder, defaults to first unet, as suggested was used in the paper

dalle2_pytorch/dalle2_pytorch.py

@@ -33,6 +33,10 @@ from rotary_embedding_torch import RotaryEmbedding
 from x_clip import CLIP
 from coca_pytorch import CoCa
 
+# constants
+
+NAT = 1. / math.log(2.)
+
 # helper functions
 
 def exists(val):
@@ -91,6 +95,9 @@ def freeze_model_and_make_eval_(model):
 
 # tensor helpers
 
+def log(t, eps = 1e-12):
+    return torch.log(t.clamp(min = eps))
+
 def l2norm(t):
     return F.normalize(t, dim = -1)
 
@@ -297,6 +304,36 @@ def noise_like(shape, device, repeat=False):
     noise = lambda: torch.randn(shape, device=device)
     return repeat_noise() if repeat else noise()
 
+def meanflat(x):
+    return x.mean(dim = tuple(range(1, len(x.shape))))
+
+def normal_kl(mean1, logvar1, mean2, logvar2):
+    return 0.5 * (-1.0 + logvar2 - logvar1 + torch.exp(logvar1 - logvar2) + ((mean1 - mean2) ** 2) * torch.exp(-logvar2))
+
+def approx_standard_normal_cdf(x):
+    return 0.5 * (1.0 + torch.tanh(((2.0 / math.pi) ** 0.5) * (x + 0.044715 * (x ** 3))))
+
+def discretized_gaussian_log_likelihood(x, *, means, log_scales, thres = 0.999):
+    assert x.shape == means.shape == log_scales.shape
+
+    centered_x = x - means
+    inv_stdv = torch.exp(-log_scales)
+    plus_in = inv_stdv * (centered_x + 1. / 255.)
+    cdf_plus = approx_standard_normal_cdf(plus_in)
+    min_in = inv_stdv * (centered_x - 1. / 255.)
+    cdf_min = approx_standard_normal_cdf(min_in)
+    log_cdf_plus = log(cdf_plus)
+    log_one_minus_cdf_min = log(1. - cdf_min)
+    cdf_delta = cdf_plus - cdf_min
+
+    log_probs = torch.where(x < -thres,
+        log_cdf_plus,
+        torch.where(x > thres,
+            log_one_minus_cdf_min,
+            log(cdf_delta)))
+
+    return log_probs
+
 def cosine_beta_schedule(timesteps, s = 0.008):
     """
     cosine schedule
@@ -1266,6 +1303,7 @@ class Unet(nn.Module):
         out_dim = None,
         dim_mults=(1, 2, 4, 8),
         channels = 3,
+        channels_out = None,
         attn_dim_head = 32,
         attn_heads = 16,
         lowres_cond = False, # for cascading diffusion - https://cascaded-diffusion.github.io/
@@ -1296,6 +1334,7 @@ class Unet(nn.Module):
         # determine dimensions
 
         self.channels = channels
+        self.channels_out = default(channels_out, channels)
 
         init_channels = channels if not lowres_cond else channels * 2 # in cascading diffusion, one concats the low resolution image, blurred, for conditioning the higher resolution synthesis
         init_dim = default(init_dim, dim // 3 * 2)
@@ -1401,11 +1440,9 @@ class Unet(nn.Module):
                 Upsample(dim_in)
             ]))
 
-        out_dim = default(out_dim, channels)
-
         self.final_conv = nn.Sequential(
             ResnetBlock(dim, dim, groups = resnet_groups[0]),
-            nn.Conv2d(dim, out_dim, 1)
+            nn.Conv2d(dim, self.channels_out, 1)
         )
 
     # if the current settings for the unet are not correct
@@ -1415,13 +1452,25 @@ class Unet(nn.Module):
         *,
         lowres_cond,
         channels,
+        channels_out,
         cond_on_image_embeds,
         cond_on_text_encodings
     ):
-        if lowres_cond == self.lowres_cond and channels == self.channels and cond_on_image_embeds == self.cond_on_image_embeds and cond_on_text_encodings == self.cond_on_text_encodings:
+        if lowres_cond == self.lowres_cond and \
+            channels == self.channels and \
+            cond_on_image_embeds == self.cond_on_image_embeds and \
+            cond_on_text_encodings == self.cond_on_text_encodings and \
+            channels_out == self.channels_out:
             return self
 
-        updated_kwargs = {'lowres_cond': lowres_cond, 'channels': channels, 'cond_on_image_embeds': cond_on_image_embeds, 'cond_on_text_encodings': cond_on_text_encodings}
+        updated_kwargs = dict(
+            lowres_cond = lowres_cond,
+            channels = channels,
+            channels_out = channels_out,
+            cond_on_image_embeds = cond_on_image_embeds,
+            cond_on_text_encodings = cond_on_text_encodings
+        )
+
         return self.__class__(**{**self._locals, **updated_kwargs})
 
     def forward_with_cond_scale(
@@ -1621,6 +1670,8 @@ class Decoder(BaseGaussianDiffusion):
         clip_denoised = True,
         clip_x_start = True,
         clip_adapter_overrides = dict(),
+        learned_variance = True,
+        vb_loss_weight = 0.001,
         unconditional = False
     ):
         super().__init__(
@@ -1659,10 +1710,18 @@ class Decoder(BaseGaussianDiffusion):
         unets = cast_tuple(unet)
         vaes = pad_tuple_to_length(cast_tuple(vae), len(unets), fillvalue = NullVQGanVAE(channels = self.channels))
 
+        # whether to use learned variance, defaults to True for the first unet in the cascade, as in paper
+
+        learned_variance = pad_tuple_to_length(cast_tuple(learned_variance), len(unets), fillvalue = False)
+        self.learned_variance = learned_variance
+        self.vb_loss_weight = vb_loss_weight
+
+        # construct unets and vaes
+
         self.unets = nn.ModuleList([])
         self.vaes = nn.ModuleList([])
 
-        for ind, (one_unet, one_vae) in enumerate(zip(unets, vaes)):
+        for ind, (one_unet, one_vae, one_unet_learned_var) in enumerate(zip(unets, vaes, learned_variance)):
             assert isinstance(one_unet, Unet)
             assert isinstance(one_vae, (VQGanVAE, NullVQGanVAE))
 
@@ -1670,12 +1729,14 @@ class Decoder(BaseGaussianDiffusion):
             latent_dim = one_vae.encoded_dim if exists(one_vae) else None
 
             unet_channels = default(latent_dim, self.channels)
+            unet_channels_out = unet_channels * (1 if not one_unet_learned_var else 2)
 
             one_unet = one_unet.cast_model_parameters(
                 lowres_cond = not is_first,
                 cond_on_image_embeds = is_first and not unconditional,
                 cond_on_text_encodings = one_unet.cond_on_text_encodings and not unconditional,
-                channels = unet_channels
+                channels = unet_channels,
+                channels_out = unet_channels_out
             )
 
             self.unets.append(one_unet)
@@ -1738,8 +1799,11 @@ class Decoder(BaseGaussianDiffusion):
         yield
         unet.cpu()
 
-    def p_mean_variance(self, unet, x, t, image_embed, text_encodings = None, text_mask = None, lowres_cond_img = None, clip_denoised = True, predict_x_start = False, cond_scale = 1.):
-        pred = unet.forward_with_cond_scale(x, t, image_embed = image_embed, text_encodings = text_encodings, text_mask = text_mask, cond_scale = cond_scale, lowres_cond_img = lowres_cond_img)
+    def p_mean_variance(self, unet, x, t, image_embed, text_encodings = None, text_mask = None, lowres_cond_img = None, clip_denoised = True, predict_x_start = False, learned_variance = False, cond_scale = 1., model_output = None):
+        pred = default(model_output, lambda: unet.forward_with_cond_scale(x, t, image_embed = image_embed, text_encodings = text_encodings, text_mask = text_mask, cond_scale = cond_scale, lowres_cond_img = lowres_cond_img))
+
+        if learned_variance:
+            pred, var_interp_frac_unnormalized = pred.chunk(2, dim = 1)
 
         if predict_x_start:
             x_recon = pred
@@ -1750,19 +1814,31 @@ class Decoder(BaseGaussianDiffusion):
             x_recon.clamp_(-1., 1.)
 
         model_mean, posterior_variance, posterior_log_variance = self.q_posterior(x_start=x_recon, x_t=x, t=t)
+
+        if learned_variance:
+            # if learned variance, posterio variance and posterior log variance are predicted by the network
+            # by an interpolation of the max and min log beta values
+            # eq 15 - https://arxiv.org/abs/2102.09672
+            min_log = extract(self.posterior_log_variance_clipped, t, x.shape)
+            max_log = extract(torch.log(self.betas), t, x.shape)
+            var_interp_frac = unnormalize_img(var_interp_frac_unnormalized)
+
+            posterior_log_variance = var_interp_frac * max_log + (1 - var_interp_frac) * min_log
+            posterior_variance = posterior_log_variance.exp()
+
         return model_mean, posterior_variance, posterior_log_variance
 
     @torch.inference_mode()
-    def p_sample(self, unet, x, t, image_embed, text_encodings = None, text_mask = None, cond_scale = 1., lowres_cond_img = None, predict_x_start = False, clip_denoised = True, repeat_noise = False):
+    def p_sample(self, unet, x, t, image_embed, text_encodings = None, text_mask = None, cond_scale = 1., lowres_cond_img = None, predict_x_start = False, learned_variance = False, clip_denoised = True, repeat_noise = False):
         b, *_, device = *x.shape, x.device
-        model_mean, _, model_log_variance = self.p_mean_variance(unet, x = x, t = t, image_embed = image_embed, text_encodings = text_encodings, text_mask = text_mask, cond_scale = cond_scale, lowres_cond_img = lowres_cond_img, clip_denoised = clip_denoised, predict_x_start = predict_x_start)
+        model_mean, _, model_log_variance = self.p_mean_variance(unet, x = x, t = t, image_embed = image_embed, text_encodings = text_encodings, text_mask = text_mask, cond_scale = cond_scale, lowres_cond_img = lowres_cond_img, clip_denoised = clip_denoised, predict_x_start = predict_x_start, learned_variance = learned_variance)
         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)))
         return model_mean + nonzero_mask * (0.5 * model_log_variance).exp() * noise
 
     @torch.inference_mode()
-    def p_sample_loop(self, unet, shape, image_embed, predict_x_start = False, clip_denoised = True, lowres_cond_img = None, text_encodings = None, text_mask = None, cond_scale = 1):
+    def p_sample_loop(self, unet, shape, image_embed, predict_x_start = False, learned_variance = False, clip_denoised = True, lowres_cond_img = None, text_encodings = None, text_mask = None, cond_scale = 1):
         device = self.betas.device
 
         b = shape[0]
@@ -1779,17 +1855,18 @@ class Decoder(BaseGaussianDiffusion):
                 cond_scale = cond_scale,
                 lowres_cond_img = lowres_cond_img,
                 predict_x_start = predict_x_start,
+                learned_variance = learned_variance,
                 clip_denoised = clip_denoised
             )
 
         return img
 
-    def p_losses(self, unet, x_start, times, *, image_embed, lowres_cond_img = None, text_encodings = None, text_mask = None, predict_x_start = False, noise = None):
+    def p_losses(self, unet, x_start, times, *, image_embed, lowres_cond_img = None, text_encodings = None, text_mask = None, predict_x_start = False, noise = None, learned_variance = False, clip_denoised = False):
         noise = default(noise, lambda: torch.randn_like(x_start))
 
         x_noisy = self.q_sample(x_start = x_start, t = times, noise = noise)
 
-        pred = unet(
+        model_output = unet(
             x_noisy,
             times,
             image_embed = image_embed,
@@ -1800,10 +1877,43 @@ class Decoder(BaseGaussianDiffusion):
             text_cond_drop_prob = self.text_cond_drop_prob,
         )
 
+        if learned_variance:
+            pred, _ = model_output.chunk(2, dim = 1)
+        else:
+            pred = model_output
+
         target = noise if not predict_x_start else x_start
 
         loss = self.loss_fn(pred, target)
-        return loss
+
+        if not learned_variance:
+            # return simple loss if not using learned variance
+            return loss
+
+        # if learning the variance, also include the extra weight kl loss
+
+        true_mean, _, true_log_variance_clipped = self.q_posterior(x_start = x_start, x_t = x_noisy, t = times)
+        model_mean, _, model_log_variance = self.p_mean_variance(unet, x = x_noisy, t = times, image_embed = image_embed, clip_denoised = clip_denoised, learned_variance = True, model_output = model_output)
+
+        # kl loss with detached model predicted mean, for stability reasons as in paper
+
+        detached_model_mean = model_mean.detach()
+
+        kl = normal_kl(true_mean, true_log_variance_clipped, detached_model_mean, model_log_variance)
+        kl = meanflat(kl) * NAT
+
+        decoder_nll = -discretized_gaussian_log_likelihood(x_start, means = detached_model_mean, log_scales = 0.5 * model_log_variance)
+        decoder_nll = meanflat(decoder_nll) * NAT
+
+        # at the first timestep return the decoder NLL, otherwise return KL(q(x_{t-1}|x_t,x_0) || p(x_{t-1}|x_t))
+
+        vb_losses = torch.where(times == 0, decoder_nll, kl)
+
+        # weight the vb loss smaller, for stability, as in the paper (recommended 0.001)
+
+        vb_loss = vb_losses.mean() * self.vb_loss_weight
+
+        return loss + vb_loss
 
     @torch.inference_mode()
     @eval_decorator
@@ -1830,7 +1940,7 @@ class Decoder(BaseGaussianDiffusion):
         img = None
         is_cuda = next(self.parameters()).is_cuda
 
-        for unet_number, unet, vae, channel, image_size, predict_x_start in tqdm(zip(range(1, len(self.unets) + 1), self.unets, self.vaes, self.sample_channels, self.image_sizes, self.predict_x_start)):
+        for unet_number, unet, vae, channel, image_size, predict_x_start, learned_variance in tqdm(zip(range(1, len(self.unets) + 1), self.unets, self.vaes, self.sample_channels, self.image_sizes, self.predict_x_start, self.learned_variance)):
 
             context = self.one_unet_in_gpu(unet = unet) if is_cuda else null_context()
 
@@ -1856,6 +1966,7 @@ class Decoder(BaseGaussianDiffusion):
                     text_mask = text_mask,
                     cond_scale = cond_scale,
                     predict_x_start = predict_x_start,
+                    learned_variance = learned_variance,
                     clip_denoised = not is_latent_diffusion,
                     lowres_cond_img = lowres_cond_img
                 )
@@ -1885,6 +1996,7 @@ class Decoder(BaseGaussianDiffusion):
         target_image_size   = self.image_sizes[unet_index]
         predict_x_start     = self.predict_x_start[unet_index]
         random_crop_size    = self.random_crop_sizes[unet_index]
+        learned_variance    = self.learned_variance[unet_index]
         b, c, h, w, device, = *image.shape, image.device
 
         check_shape(image, 'b c h w', c = self.channels)
@@ -1922,7 +2034,7 @@ class Decoder(BaseGaussianDiffusion):
             if exists(lowres_cond_img):
                 lowres_cond_img = vae.encode(lowres_cond_img)
 
-        return self.p_losses(unet, image, times, image_embed = image_embed, text_encodings = text_encodings, text_mask = text_mask, lowres_cond_img = lowres_cond_img, predict_x_start = predict_x_start)
+        return self.p_losses(unet, image, times, image_embed = image_embed, text_encodings = text_encodings, text_mask = text_mask, lowres_cond_img = lowres_cond_img, predict_x_start = predict_x_start, learned_variance = learned_variance)
 
 # main class
 
@@ -1972,4 +2084,3 @@ class DALLE2(nn.Module):
             return images[0]
 
         return images
-

setup.py

@@ -10,7 +10,7 @@ setup(
       'dream = dalle2_pytorch.cli:dream'
     ],
   },
-  version = '0.2.12',
+  version = '0.2.14',
   license='MIT',
   description = 'DALL-E 2',
   author = 'Phil Wang',