Add Apple Silicon (MPS) support

[provos]

Adds MPS device support for both image and video predictors on Apple Silicon.

Changes:

• Add get_default_device() utility that detects MPS availability
• Fix device mismatches (coords cache, freqs_cis cache)
• Add MPS workaround for complex tensor repeat() in RoPE
• Make torch._assert_async conditional on CUDA
• Fix MPS memory leak in video predictor via synchronization points

Performance of the Video predictor:

• ~3x faster than CPU
• Runs with ~38GB peak memory. This is due to the way that MPS caches graphs. Before adding the synchronization points, running the video predictor would consume all available memory.