fix: enable MPS for the PyTorch Qwen3-TTS/CustomVoice engines on Apple Silicon

get_torch_device() already supports allow_mps, and other engines
(luxtts_backend.py, qwen_llm_backend.py) already pass allow_mps=True —
but PyTorchTTSBackend, PyTorchSTTBackend, and QwenCustomVoiceBackend
never did, so any Apple Silicon install that ends up on the PyTorch
backend (e.g. MLX unavailable, or before this PR's justfile fix)
silently ran these three on plain CPU instead of falling back to
PyTorch+MPS.

This turned out to matter more than expected: benchmarking on M4
hardware, PyTorch+MPS generation (RTF ~1.7x) was faster than this
same PR's MLX thread-fix (RTF ~3.4-4.3x) for the same qwen engine and
model size, and faster than a CUDA RTX 2080 Ti (RTF ~2.3-3.1x) too.
I'm not proposing changing the default backend selection here — one
engine, one model size, one machine is not enough data for that, and
MLX-Whisper is reportedly faster than PyTorch-CPU for STT — but MPS
should clearly be an option rather than silently absent.

Verified on M4: PyTorchTTSBackend()._get_device() and
QwenCustomVoiceBackend()._get_device() both now return "mps" (were
"cpu").

Author: AlexStephenLytton

backend/backends/pytorch_backend.py

@@ -34,7 +34,7 @@ def __init__(self, model_size: str = "1.7B"):
 
     def _get_device(self) -> str:
         """Get the best available device."""
-        return get_torch_device(allow_xpu=True, allow_directml=True)
+        return get_torch_device(allow_xpu=True, allow_directml=True, allow_mps=True)
 
     def is_loaded(self) -> bool:
         """Check if model is loaded."""
@@ -256,7 +256,7 @@ def __init__(self, model_size: str = "base"):
 
     def _get_device(self) -> str:
         """Get the best available device."""
-        return get_torch_device(allow_xpu=True, allow_directml=True)
+        return get_torch_device(allow_xpu=True, allow_directml=True, allow_mps=True)
 
     def is_loaded(self) -> bool:
         """Check if model is loaded."""

backend/backends/qwen_custom_voice_backend.py

@@ -65,7 +65,7 @@ def __init__(self, model_size: str = "1.7B"):
         self._current_model_size: Optional[str] = None
 
     def _get_device(self) -> str:
-        return get_torch_device(allow_xpu=True, allow_directml=True)
+        return get_torch_device(allow_xpu=True, allow_directml=True, allow_mps=True)
 
     def is_loaded(self) -> bool:
         return self.model is not None