[SD3 dreambooth lora] smol fix to checkpoint saving (huggingface#9993)

* smol change to fix checkpoint saving & resuming (as done in train_dreambooth_sd3.py)

* style

* modify comment to explain reasoning behind hidden size check

Author: linoytsaban

examples/dreambooth/train_dreambooth_lora_sd3.py

@@ -1294,10 +1294,13 @@ def save_model_hook(models, weights, output_dir):
             for model in models:
                 if isinstance(model, type(unwrap_model(transformer))):
                     transformer_lora_layers_to_save = get_peft_model_state_dict(model)
-                elif isinstance(model, type(unwrap_model(text_encoder_one))):
-                    text_encoder_one_lora_layers_to_save = get_peft_model_state_dict(model)
-                elif isinstance(model, type(unwrap_model(text_encoder_two))):
-                    text_encoder_two_lora_layers_to_save = get_peft_model_state_dict(model)
+                elif isinstance(model, type(unwrap_model(text_encoder_one))):  # or text_encoder_two
+                    # both text encoders are of the same class, so we check hidden size to distinguish between the two
+                    hidden_size = unwrap_model(model).config.hidden_size
+                    if hidden_size == 768:
+                        text_encoder_one_lora_layers_to_save = get_peft_model_state_dict(model)
+                    elif hidden_size == 1280:
+                        text_encoder_two_lora_layers_to_save = get_peft_model_state_dict(model)
                 else:
                     raise ValueError(f"unexpected save model: {model.__class__}")