Fix the input pixel format when using GPU video encoder

[mthrok]

StreamWriter's encoding pipeline looks like the following

1. convert tensor to AVFrame
2. pass AVFrame to AVFilter
3. pass the resulting AVFrame to AVCodecContext (encoder) and AVFormatContext (muxer)

When dealing with CUDA tensor, the AVFilter becomes no-op, as we have not added support for CUDA-compatible filters.

When CUDA frame is passed, the existing solution passes the software pixel format to AVFilter, which issues warning later as what AVFilter sees is AV_PIX_FMT_CUDA.

Since the filter itself is no-op, it functions as expected. But this commit fixes it.

See #3317

[pytorch-bot]

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❌ 1 New Failure, 12 Pending, 1 Unrelated Failure

As of commit d8cdced:

NEW FAILURE - The following job has failed:

• build (3.8) (gh)

BROKEN TRUNK - The following job failed but were present on the merge base a7fea8a:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• unittests-windows-gpu / windows-job (gh)

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@mthrok merged this pull request in 30afaa9.

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