Use average frame rate for video metadata

[mavenlin]

Summary

• populate video average_fps_from_header from FFmpeg’s AVStream::avg_frame_rate instead of r_frame_rate
• add a 29 KB variable-frame-rate regression fixture derived from the repository’s existing NASA video

Why

r_frame_rate is FFmpeg’s estimate of the lowest-frequency regular grid that can represent all frame timestamps accurately, the coarsest sufficient grid.

The regression test first asserts that ffprobe reports different values for the two fields, then compares TorchCodec against ffprobe’s avg_frame_rate and away from its r_frame_rate.

Fixture generation

The fixture uses the 480×270 stream from test/resources/nasa_13013.mp4, preserving source timestamps while selecting every second frame for the first 60 source frames and every third frame afterward.

ffmpeg -i test/resources/nasa_13013.mp4 -map 0:v:1 -t 4 \
  -vf "select=if(lt(n\,60)\,not(mod(n\,2))\,not(mod(n\,3)))" \
  -fps_mode vfr -c:v libx264 -pix_fmt yuv420p -an \
  test/resources/nasa_13013_vfr.mp4

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/meta-pytorch/torchcodec/1628

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