Make zeroing prompt embeds for Mochi Pipeline configurable (#10284)

update

src/diffusers/pipelines/mochi/pipeline_mochi.py

@@ -188,6 +188,7 @@ class MochiPipeline(DiffusionPipeline, Mochi1LoraLoaderMixin):
         text_encoder: T5EncoderModel,
         tokenizer: T5TokenizerFast,
         transformer: MochiTransformer3DModel,
+        force_zeros_for_empty_prompt: bool = False,
     ):
         super().__init__()
 
@@ -205,10 +206,11 @@ class MochiPipeline(DiffusionPipeline, Mochi1LoraLoaderMixin):
 
         self.video_processor = VideoProcessor(vae_scale_factor=self.vae_spatial_scale_factor)
         self.tokenizer_max_length = (
-            self.tokenizer.model_max_length if hasattr(self, "tokenizer") and self.tokenizer is not None else 77
+            self.tokenizer.model_max_length if hasattr(self, "tokenizer") and self.tokenizer is not None else 256
         )
         self.default_height = 480
         self.default_width = 848
+        self.register_to_config(force_zeros_for_empty_prompt=force_zeros_for_empty_prompt)
 
     def _get_t5_prompt_embeds(
         self,
@@ -236,7 +238,11 @@ class MochiPipeline(DiffusionPipeline, Mochi1LoraLoaderMixin):
         text_input_ids = text_inputs.input_ids
         prompt_attention_mask = text_inputs.attention_mask
         prompt_attention_mask = prompt_attention_mask.bool().to(device)
-        if prompt == "" or prompt[-1] == "":
+
+        # The original Mochi implementation zeros out empty negative prompts
+        # but this can lead to overflow when placing the entire pipeline under the autocast context
+        # adding this here so that we can enable zeroing prompts if necessary
+        if self.config.force_zeros_for_empty_prompt and (prompt == "" or prompt[-1] == ""):
             text_input_ids = torch.zeros_like(text_input_ids, device=device)
             prompt_attention_mask = torch.zeros_like(prompt_attention_mask, dtype=torch.bool, device=device)