Fix Stream Generator on MacOS

[gravityrail]

Quick fix for streaming responses on macOS. It was crashing somewhere in pytorch. Unfortunately I can't find the original post where I found this patch to give credit - too many tabs open! But it works for me.

For this to work, you must run with PYTORCH_ENABLE_MPS_FALLBACK=1 and call model.to(mps_device) on the XttsModel

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[eginhard]

The crash occurs on any OS with transformers>=4.41. This fix was originally shared by @pseudotensor in idiap#31. But I just opened idiap#46 in our fork (since this repo is not updated anymore) which more generally aligns the streaming code to the upstream transformers implementation again, so it should be more resilient to future updates. It will be available in the next release.