fix: unscale fp16 gradient problem & potential error (#6086) (#6231)

Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

examples/text_to_image/train_text_to_image_lora_sdxl.py

@@ -640,6 +640,17 @@ def main(args):
         text_encoder_one.add_adapter(text_lora_config)
         text_encoder_two.add_adapter(text_lora_config)
 
+    # Make sure the trainable params are in float32.
+    if args.mixed_precision == "fp16":
+        models = [unet]
+        if args.train_text_encoder:
+            models.extend([text_encoder_one, text_encoder_two])
+        for model in models:
+            for param in model.parameters():
+                # only upcast trainable parameters (LoRA) into fp32
+                if param.requires_grad:
+                    param.data = param.to(torch.float32)
+
     # create custom saving & loading hooks so that `accelerator.save_state(...)` serializes in a nice format
     def save_model_hook(models, weights, output_dir):
         if accelerator.is_main_process:
@@ -1187,6 +1198,9 @@ def main(args):
         torch.cuda.empty_cache()
 
         # Final inference
+        # Make sure vae.dtype is consistent with the unet.dtype
+        if args.mixed_precision == "fp16":
+            vae.to(weight_dtype)
         # Load previous pipeline
         pipeline = StableDiffusionXLPipeline.from_pretrained(
             args.pretrained_model_name_or_path,