fix: macOS MPS support improvements (dtype, xpu safety, memory)

[takayamaekawa]

Summary

macOS (Apple Silicon) でVibeVoiceを動作させる際のバグ修正と改善です。

• torch_dtype 引数名の修正: from_pretrained に dtype= を渡していたのを正しい torch_dtype= に修正
• torch.backends.xpu の安全なチェック: macOSでは xpu 属性が存在しないため AttributeError が発生していた。hasattr チェックを追加して修正
• MPS dtype を float16 に変更: float32 のままだとメモリ使用量が約28GBとなりM-series Mac上ではOOMになるケースがあった。float16 にすることで約14GBに削減
• _convert_dtype_to_string をネスト辞書に対応: ネストされた dict 内の torch.dtype が変換されずJSON直列化エラーが発生していた問題を修正 (ref: Object of type dtype is not JSON serializable #199)

Test plan

• Apple Silicon Mac (MPS) で demo/vibevoice_asr_inference_from_file.py が正常に動作することを確認
• --device mps 指定時にメモリ使用量が float16 で約14GB以内に収まることを確認
• CUDA / CPU 環境での動作に影響がないことを確認

🤖 Generated with Claude Code

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[rickthomasjr]

Note from automated review

macOS MPS support improvements (dtype, xpu safety, memory). I don't have a macOS/MPS environment to verify these changes. The diff looks like targeted fixes for Metal Performance Shaders compatibility.

Verdict: Left for Rick or an Apple Silicon specialist to confirm. If tested on MPS, ready to merge.