feat(pai): replace imageio-ffmpeg with PyNvVideoCodec for H.264 decode

[janickm]

Summary

• Replaces imageio-ffmpeg (BSD-2-Clause wrapper bundling GPL ffmpeg binary) with PyNvVideoCodec (MIT license, NVIDIA-maintained) for video decoding in the PAI data converter
• Uses nvc.SimpleDecoder with CPU host memory output and RGB color format to match existing pipeline expectations
• Eliminates ffmpeg redistribution licensing concerns for downstream container builds
• Requires NVIDIA GPU (Turing+) for hardware-accelerated H.264 decode via NVDEC

Testing

Verified end-to-end with:

bazel run //tools/data_converter/pai -- \
    --output-dir=/tmp/pai \
    --camera-id=camera_front_wide_120fov \
    pai-stream-v4 \
    --hf-token=<token> \
    --clip-id=4f14b8f4-4c16-48ce-aa42-02add7cd9f4d \
    --duration-sec=1

Conversion completed successfully with GPU-accelerated decode at ~63 frames/sec.