Fix Apple Silicon (MPS) watermarking crash & enable MPS device

[Thearchitectoffire]

The MisoTTS watermark (silentcipher) relies on aten::unfold_backward during its inverse STFT, which is not natively supported by the MPS backend on Apple Silicon Macs and causes the generation to crash at the very end. Setting PYTORCH_ENABLE_MPS_FALLBACK=1 in the environment variables fixes this crash by allowing just that operation to fall back to the CPU while keeping the heavy generation on the GPU. I also enabled MPS by default in run_misotts.py since the float64 limitation doesn't seem to stop the generation from working with this fallback.

[ded-furby]

I think the fallback env var is currently set too late to fix the default run_misotts.py path. run_misotts.py imports torch before it imports generator, so by the time generator.py executes os.environ.setdefault("PYTORCH_ENABLE_MPS_FALLBACK", "1"), PyTorch has already initialized without the fallback flag.

That means users who run the documented entrypoint can still hit the same MPS crash even though the PR adds the env var. Moving the flag into run_misotts.py before import torch (or documenting an external env export) should make the fix effective for the common path.