torchcodec-xpu: implement a test to verify CPU fallback

[dvrogozh]

Torchcodec has a mechanism to report CPU fallback. See:

• https://github.com/meta-pytorch/torchcodec/blob/d1a1197d985804f55f77406a176abc796960591b/test/test_decoders.py#L2003
• https://github.com/meta-pytorch/torchcodec/blob/d1a1197d985804f55f77406a176abc796960591b/src/torchcodec/decoders/_video_decoder.py#L26

We also might need to generalize approach in Torchcodec before fully implementing this.

CC: @eromomon

[dvrogozh]

Test for CPU fallback added via #49. Reporting of CPU fallback still needs to be looked at. It might need separate test.