fix: pop second_per_grid_ts after mrope

[ccilery]

What does this PR do?

Pop second_per_grid_ts after mrope, otherwise second_per_grid_ts in get_rope_index always be None and fps for input videos always 1.

#1620

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[gemini-code-assist]

Code Review

This pull request addresses a bug where second_per_grid_ts was being removed from model_inputs before its use in the get_rope_index function for m-RoPE calculation. This caused the fps for input videos to default to 1 incorrectly. The fix relocates the pop operation to after the m-RoPE calculation, ensuring the correct value is available when needed. The change is correct and effectively resolves the bug.