remove tests from collection

[mollyxu]

Remove skipped tests from collection in internal environment

Prevent internal CI failures from skipped tests by excluding skipped tests from collection in internal environment.

TorchCodec tests were flaky / broken internal because:

• Tests were using runtime pytest.skip() calls, so they were collected but then skipped during execution

Changes Made

1. Added @needs_ffmpeg_cli marker to tests that need ffmpeg-cli

• test_ops.py::TestVideoDecoderOps::test_seek_mode_custom_frame_mappings
• test_ops.py::TestVideoDecoderOps:: test_seek_mode_custom_frame_mappings_fails
• test_encoders.py::TestAudioEncoder::test_against_cli
• test_encoders.py::TestAudioEncoder::test_video_encoder_against_ffmpeg_cli
• test_encoders.py::TestAudioEncoder::test_codec_parameter_utilized
• test_encoders.py::TestAudioEncoder::test_codec_spec_vs_impl_equivalence
• test_encoders.py::TestAudioEncoder::test_extra_options_utilized
• test_decoders.py::TestVideoDecoder::test_custom_frame_mappings_json_and_bytes
• test_decoders.py::TestVideoDecoder::test_custom_frame_mappings_init_fails

2. Converted runtime skips to collection-time skips

Replaced runtime pytest.skip() calls with pytest.param(..., marks=pytest.mark.skipif(...)) for:

• FFmpeg version compatibility checks (e.g., FFmpeg 4 incompatibility with wav files)
• Platform-specific codec availability (e.g., webm codec unavailable on Windows with certain FFmpeg versions)

3. Updated conftest.py to exclude tests internally

Modified pytest_collection_modifyitems hook to exclude tests from collection when running internally if they have:

• @needs_ffmpeg_cli marker
• @pytest.mark.skip() decorator
• @pytest.mark.skipif() decorator

[meta-codesync]

@mollyxu has imported this pull request. If you are a Meta employee, you can view this in D87512192.