New Optical Flow OP & Allow to save the computed optical flows

[HYLcool]

As the title says.

[gemini-code-assist]

Summary of Changes

Hello @HYLcool, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant enhancement to the video motion score filters by allowing users to persist the computed optical flow data. Previously, optical flow was an internal calculation used solely for determining motion scores. With this change, users can now opt to save this detailed motion information directly into the dataset's metadata, providing greater flexibility for downstream analysis or further processing. This feature is implemented across both the standard "VideoMotionScoreFilter" and the "VideoMotionScoreRaftFilter".

Highlights

• Optical Flow Output: Introduced new parameters "if_output_optical_flow" and "optical_flow_key" to "VideoMotionScoreFilter" and "VideoMotionScoreRaftFilter" to enable saving computed optical flows.
• Metadata Storage: When enabled, the computed optical flows for each video are stored in the dataset's metadata under the specified "optical_flow_key" (defaulting to "MetaKeys.video_optical_flow").
• New Constant: Added "MetaKeys.video_optical_flow" to "data_juicer/utils/constant.py" for consistent key naming.
• Test Coverage: New unit tests were added for both "VideoMotionScoreFilter" and "VideoMotionScoreRaftFilter" to ensure the correct functionality of the optical flow output feature.

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[gemini-code-assist]

Code Review

This pull request adds a useful feature to optionally save computed optical flows in two video motion score filters. The implementation is mostly correct, but I've identified a critical issue that could lead to a crash when processing videos with few frames or corrupted data. Additionally, there's a minor typo in a docstring, and the new tests could be more robust. I've provided specific suggestions to address these points.