[Training] SD3 training fixes (#8917)

* SD3 training fixes
Co-authored-by: bghira <59658056+bghira@users.noreply.github.com>

* rewrite noise addition part to respect the eqn.

* styler

* Update examples/dreambooth/README_sd3.md
Co-authored-by: Kashif Rasul <kashif.rasul@gmail.com>

---------
Co-authored-by: bghira <59658056+bghira@users.noreply.github.com>
Co-authored-by: Kashif Rasul <kashif.rasul@gmail.com>

examples/dreambooth/README_sd3.md

@@ -183,4 +183,6 @@ accelerate launch train_dreambooth_lora_sd3.py \
 
 ## Other notes
 
-We default to the "logit_normal" weighting scheme for the loss following the SD3 paper. Thanks to @bghira for helping us discover that for other weighting schemes supported from the training script, training may incur numerical instabilities.
+1. We default to the "logit_normal" weighting scheme for the loss following the SD3 paper. Thanks to @bghira for helping us discover that for other weighting schemes supported from the training script, training may incur numerical instabilities.
\ No newline at end of file
+2. Thanks to `bghira`, `JinxuXiang`, and `bendanzzc` for helping us discover a bug in how VAE encoding was being done previously. This has been fixed in [#8917](https://github.com/huggingface/diffusers/pull/8917).
+3. Additionally, we now have the option to control if we want to apply preconditioning to the model outputs via a `--precondition_outputs` CLI arg. It affects how the model `target` is calculated as well. 
\ No newline at end of file

examples/dreambooth/train_dreambooth_lora_sd3.py

@@ -523,6 +523,13 @@ def parse_args(input_args=None):
         default=1.29,
         help="Scale of mode weighting scheme. Only effective when using the `'mode'` as the `weighting_scheme`.",
     )
+    parser.add_argument(
+        "--precondition_outputs",
+        type=int,
+        default=1,
+        help="Flag indicating if we are preconditioning the model outputs or not as done in EDM. This affects how "
+        "model `target` is calculated.",
+    )
     parser.add_argument(
         "--optimizer",
         type=str,
@@ -1636,7 +1643,7 @@ def main(args):
 
                 # Convert images to latent space
                 model_input = vae.encode(pixel_values).latent_dist.sample()
-                model_input = model_input * vae.config.scaling_factor
+                model_input = (model_input - vae.config.shift_factor) * vae.config.scaling_factor
                 model_input = model_input.to(dtype=weight_dtype)
 
                 # Sample noise that we'll add to the latents
@@ -1656,8 +1663,9 @@ def main(args):
                 timesteps = noise_scheduler_copy.timesteps[indices].to(device=model_input.device)
 
                 # Add noise according to flow matching.
+                # zt = (1 - texp) * x + texp * z1
                 sigmas = get_sigmas(timesteps, n_dim=model_input.ndim, dtype=model_input.dtype)
-                noisy_model_input = sigmas * noise + (1.0 - sigmas) * model_input
+                noisy_model_input = (1.0 - sigmas) * model_input + sigmas * noise
 
                 # Predict the noise residual
                 model_pred = transformer(
@@ -1670,6 +1678,7 @@ def main(args):
 
                 # Follow: Section 5 of https://arxiv.org/abs/2206.00364.
                 # Preconditioning of the model outputs.
+                if args.precondition_outputs:
                     model_pred = model_pred * (-sigmas) + noisy_model_input
 
                 # these weighting schemes use a uniform timestep sampling
@@ -1677,7 +1686,10 @@ def main(args):
                 weighting = compute_loss_weighting_for_sd3(weighting_scheme=args.weighting_scheme, sigmas=sigmas)
 
                 # flow matching loss
+                if args.precondition_outputs:
                     target = model_input
+                else:
+                    target = noise - model_input
 
                 if args.with_prior_preservation:
                     # Chunk the noise and model_pred into two parts and compute the loss on each part separately.

examples/dreambooth/train_dreambooth_sd3.py

@@ -494,6 +494,13 @@ def parse_args(input_args=None):
         default=1.29,
         help="Scale of mode weighting scheme. Only effective when using the `'mode'` as the `weighting_scheme`.",
     )
+    parser.add_argument(
+        "--precondition_outputs",
+        type=int,
+        default=1,
+        help="Flag indicating if we are preconditioning the model outputs or not as done in EDM. This affects how "
+        "model `target` is calculated.",
+    )
     parser.add_argument(
         "--optimizer",
         type=str,
@@ -1549,7 +1556,7 @@ def main(args):
 
                 # Convert images to latent space
                 model_input = vae.encode(pixel_values).latent_dist.sample()
-                model_input = model_input * vae.config.scaling_factor
+                model_input = (model_input - vae.config.shift_factor) * vae.config.scaling_factor
                 model_input = model_input.to(dtype=weight_dtype)
 
                 # Sample noise that we'll add to the latents
@@ -1569,8 +1576,9 @@ def main(args):
                 timesteps = noise_scheduler_copy.timesteps[indices].to(device=model_input.device)
 
                 # Add noise according to flow matching.
+                # zt = (1 - texp) * x + texp * z1
                 sigmas = get_sigmas(timesteps, n_dim=model_input.ndim, dtype=model_input.dtype)
-                noisy_model_input = sigmas * noise + (1.0 - sigmas) * model_input
+                noisy_model_input = (1.0 - sigmas) * model_input + sigmas * noise
 
                 # Predict the noise residual
                 if not args.train_text_encoder:
@@ -1598,13 +1606,18 @@ def main(args):
 
                 # Follow: Section 5 of https://arxiv.org/abs/2206.00364.
                 # Preconditioning of the model outputs.
+                if args.precondition_outputs:
                     model_pred = model_pred * (-sigmas) + noisy_model_input
+
                 # these weighting schemes use a uniform timestep sampling
                 # and instead post-weight the loss
                 weighting = compute_loss_weighting_for_sd3(weighting_scheme=args.weighting_scheme, sigmas=sigmas)
 
                 # flow matching loss
+                if args.precondition_outputs:
                     target = model_input
+                else:
+                    target = noise - model_input
 
                 if args.with_prior_preservation:
                     # Chunk the noise and model_pred into two parts and compute the loss on each part separately.