fix: remove empty multimodal input to avoid invalid uuid check in vLLM (#6853)

Signed-off-by: Guan Luo <41310872+GuanLuo@users.noreply.github.com>

components/src/dynamo/vllm/multimodal_handlers/multimodal_pd_worker_handler.py

@@ -201,8 +201,19 @@ class MultimodalPDWorkerHandler(BaseWorkerHandler):
                 )
             if image_embeds is not None:
                 request.embeddings_shape = list(image_embeds.shape)
+        # prune empty multimodal data, vLLM will expect multi_modal_uuids if the mm items are empty
+        # i.e. ValueError: multi_modal_data['image'] is empty but multi_modal_uuids['image'] is missing.
+        for key, value in multi_modal_data.items():
+            if not isinstance(value, torch.Tensor):
+                if not value:
+                    del multi_modal_data[key]
+                else:
+                    # [gluo FIXME] should be mindful to default dict, move this evaluation logic to here
+                    # so that we don't accidentally add empty keys to the dict which causes vLLM misbehavior
+                    logger.debug(
+                        f"Prepared multimodal data size: {len(multi_modal_data[key])}"
+                    )
 
-        logger.debug(f"Prepared multimodal data size: {len(multi_modal_data['image'])}")
         logger.debug("Multimodal data keys: %s", list(multi_modal_data.keys()))
 
     @staticmethod