fix(template): guard MiniCPM-V/mPLUG-Owl3 video sampling against 0-fps metadata

[he-yufeng]

What

load_video_minicpmv_mplug_owl3 picks frames with

sample_fps = round(vr.get_avg_fps() / 1)  # FPS
frame_idx = [i for i in range(0, len(vr), sample_fps)]

decord's get_avg_fps() returns 0.0 when the container has no / broken fps metadata (and a very low sub-1 fps rounds to 0 as well), so sample_fps becomes 0 and range(0, len(vr), 0) raises ValueError: range() arg 3 must not be zero before a single frame is read. The MiniCPM-V and mPLUG-Owl3 templates call this for any user-supplied video (swift/template/templates/minicpm.py, swift/template/templates/mplug.py), and there is no upstream fps check, so an ordinary video with missing fps metadata crashes encoding. The same file already treats fps == 0 as a real case elsewhere (_video_get_metadata_local uses fps if fps > 0 else 0).

Fix

Clamp sample_fps to at least 1 so a 0-fps video falls back to sampling every frame, which the existing max_num_frames uniform_sample then downsamples. Normal videos (fps >= 1) are unaffected.

Verification

decord/torch are not installable in my environment, so I reproduced the exact sampling logic standalone:

avg fps	before	after
25.0	4 frames	4 frames
1.0	4 frames	4 frames
0.4	ValueError: range() arg 3 must not be zero	4 frames
0.0	ValueError: range() arg 3 must not be zero	4 frames

The added test_load_video_minicpmv_handles_zero_fps mocks a decord VideoReader whose get_avg_fps() returns 0.0 and asserts load_video_minicpmv_mplug_owl3 returns frames instead of raising (fails before this change, passes after). It needs no real video or GPU.

[gemini-code-assist]

Code Review

This pull request ensures that sample_fps is clamped to a minimum value of 1 in vision_utils.py, preventing a zero-step range error when processing videos with 0 FPS. A corresponding unit test, test_load_video_minicpmv_handles_zero_fps, has been added to verify this fix. There are no review comments, and I have no feedback to provide.

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