Make the cv2 dependency optional

[cmpute]

Purpose

The latest code will requires the cv2 dependency (no matter command you run, because it's imported by default), and it's completely not necessary (it's only used in benchmark). This PR move cv2 inside the function it's called, to prevent cv2 to be imported by default.

The motivation to remove cv2 is that, it's hard to maintain opencv package consistency if you run vllm along with other related packages.

Test Plan

None

Test Result

None

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[gemini-code-assist]

Code Review

This pull request correctly makes the cv2 dependency optional by moving its import from the top level of vllm/benchmarks/datasets.py to within the generate_synthetic_video function, which is the only place it's used in that file. This change is well-aligned with the goal of preventing cv2 from being a required dependency for all vLLM commands. The implementation is clean and follows a common pattern for handling optional dependencies, improving the project's dependency management without introducing any issues. The change is approved.

[BloodAxe]

The latest code will requires the cv2 dependency (no matter command you run, because it's imported by default), and it's completely not necessary

This statement is not correct.
OpenCV used in video reading backend for multimodal models.

[cmpute]

The latest code will requires the cv2 dependency (no matter command you run, because it's imported by default), and it's completely not necessary

This statement is not correct.
OpenCV used in video reading backend for multimodal models.

It's not rigorously correct, but at least here is the only place where cv2 was explicitly imported.

[cmpute]

@ZJY0516 Could you mark this MR as reviewed please? I have updated the commit with the sign-off message.