Add video encoding support with SYCL RGB→NV12 kernel and CPU fallback

[eromomon]

Adds a hardware video encoding path to the torchcodec-xpu plugin (supersedes #58)

What changes:

• XpuDeviceInterface.{h,cpp} — implements three encoder overrides:

1. • get_encoding_pixel_format (pinned to NV12).
2. • setup_hardware_frame_context_for_encoding (VAAPI hw_frames_ctx, BT.709 + JPEG-range defaults, B-frames disabled, low_power=0, quality=1).

3. • convert_tensor_to_av_frame_for_encoding dispatching to:

   • • -SYCL fast path — exports the VA surface as DMA-BUF, imports via Level Zero USM, runs convertRGBToNV12 directly on the surface (handles linear + Intel Tile-Y, and both NV12 export layouts on iHD/i915).
   • • -CPU fallback — GBRP→NV12 via libswscale, then av_hwframe_transfer_data onto the VA surface. Used when SYCL is disabled or FP64 is unavailable (DG2 / ATS-M).
4. • find_codec extended to resolve VAAPI encoders, with H.264 / HEVC / AV1 fallback when the requested codec id has no VAAPI encoder.

• ColorConversionKernel.{h,cpp} — adds convertRGBToNV12 SYCL kernel (BT.601 / BT.709, limited + full range); get_tile_offset reused for encode.

• patches/0002-Add-XPU-support-to-video-encoder-tests.patch — parametrizes upstream torchcodec encoder tests over CUDA + XPU, and primes the encoder with one frame in test_write_frames_different_devices_errors (VAAPI segfaults on empty flush; NVENC doesn't). Depends on 0001-Add-XPU-support-to-tests.patch.

Compatibility
No change to decode behavior, plugin loading, or public API.

Testing
Encoder test suite (patches 0001 + 0002) passes on Intel Arc / Battlemage.

[Copilot]

Pull request overview

Adds an Intel XPU (VAAPI) hardware video encoding path to the torchcodec-xpu plugin, including an on-device SYCL RGB→NV12 conversion kernel with a CPU/libswscale fallback, and updates upstream TorchCodec encoder tests (via patch) to run on XPU as well as CUDA.

Changes:

• Implement XPU encoding overrides in XpuDeviceInterface (pixel format selection, VAAPI hw_frames_ctx setup, tensor→AVFrame upload path with SYCL fast-path + CPU fallback).
• Add SYCL convertRGBToNV12 kernel supporting BT.601/BT.709 and full/limited range, including tiled (Intel Tile-Y) surface handling.
• Extend/parameterize upstream encoder tests (patch) to exercise XPU and avoid VAAPI flush-on-empty segfault by priming with a frame.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
packages/torchcodec-xpu/src/torchcodec_xpu/XpuDeviceInterface.h	Declares XPU encoding overrides and helper conversion entry points.
packages/torchcodec-xpu/src/torchcodec_xpu/XpuDeviceInterface.cpp	Implements VAAPI frames context setup, VAAPI encoder discovery logic, and SYCL/CPU tensor→NV12 surface conversion.
packages/torchcodec-xpu/src/torchcodec_xpu/ColorConversionKernel.h	Exposes the new SYCL RGB→NV12 conversion API.
packages/torchcodec-xpu/src/torchcodec_xpu/ColorConversionKernel.cpp	Implements RGB→NV12 SYCL kernel and supporting YUV conversion matrices.
packages/torchcodec-xpu/patches/0002-Add-XPU-support-to-video-encoder-tests.patch	Updates upstream TorchCodec encoder tests to include XPU device coverage and VAAPI-specific priming behavior.

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[dvrogozh]

@eromomon , please, rebase on top of latest main and resolve conflicts.

[dvrogozh]

Last concern I have on this PR is that non-SYCL path might not be working:

USE_SYCL_KERNELS=0 TORCHCODEC_XPU_LOG_LEVEL=4 pytest -rsf --capture=no test/test_encoders.py::TestEncoder::test_add_video_and_encode_frames[xpu-to_file-mp4]

...
AssertionError: Expected at least 96% of values to be within atol=2, but only 21.34965705871582% were.