[data] fix: keyword video_metadata (verl-project#5177)

### What does this PR do?

This PR is to fix the bug of wrong keyword for video meta data.
Current code `video_metadatas` is a typo and inconsistent with
`transformers` processor code.
E.g.

https://github.com/huggingface/transformers/blob/0fa2c2f06f91129b89462e0136c0a25618e1a31b/src/transformers/models/qwen3_vl/processing_qwen3_vl.py#L116

https://github.com/huggingface/transformers/blob/0fa2c2f06f91129b89462e0136c0a25618e1a31b/src/transformers/models/qwen2_5_vl/processing_qwen2_5_vl.py#L102

https://github.com/huggingface/transformers/blob/0fa2c2f06f91129b89462e0136c0a25618e1a31b/src/transformers/models/glm4v/processing_glm4v.py#L99

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Author: sophiayyya

verl/experimental/agent_loop/agent_loop.py

@@ -286,7 +286,7 @@ async def apply_chat_template(
                 text=[raw_prompt],
                 images=images,
                 videos=videos,
-                video_metadatas=video_metadatas,
+                video_metadata=video_metadatas,
                 return_tensors="pt",
                 do_sample_frames=False,
             )
@@ -647,7 +647,7 @@ def _compute_multi_modal_inputs(self, output, input_ids) -> dict[str, torch.Tens
             text=[current_text],
             images=images,
             videos=videos,
-            video_metadatas=video_metadatas,
+            video_metadata=video_metadatas,
             return_tensors="pt",
             do_sample_frames=False,
         )