fix: Use Optional[T] for nullable type annotation defaults

[kuishou68]

Summary

This PR fixes incorrect type annotations where bare types (int, str) are used with None as default value. Per PEP 484, these should be Optional[T] = None.

Changes

• kv_channels: int = None → kv_channels: Optional[int] = None (config.py)
• out_dim: int = None → out_dim: Optional[int] = None (local_dit.py)
• cache_dir: str = None → cache_dir: Optional[str] = None (core.py)
• prompt_wav_path: str = None → prompt_wav_path: Optional[str] = None (core.py)
• prompt_text: str = None → prompt_text: Optional[str] = None (core.py)
• reference_wav_path: str = None → reference_wav_path: Optional[str] = None (core.py)

Files Modified

1. src/voxcpm/modules/minicpm4/config.py — add Optional import, fix kv_channels annotation
2. src/voxcpm/modules/locdit/local_dit.py — add Optional import, fix out_dim annotation
3. src/voxcpm/core.py — fix cache_dir, prompt_wav_path, prompt_text, reference_wav_path annotations

Closes #324

[a710128]

Thanks for the cleanup! The changes themselves look safe and annotation-only, and the lightweight checks passed on my side:

• python3 -m pytest tests/test_cli.py -> passed
• python3 -m compileall -q src/voxcpm -> passed

One thing I noticed is that the conversion is still a bit incomplete. Since the PR title is about using Optional[T] for nullable defaults, there are a few closely related cases that would be good to update as well:

• src/voxcpm/modules/locdit/local_dit_v2.py: out_dim: int = None mirrors the updated local_dit.py implementation.
• src/voxcpm/model/voxcpm.py and src/voxcpm/model/voxcpm2.py: config fields like kv_channels: int = None remain unchanged.
• src/voxcpm/model/voxcpm2.py: prompt-related nullable string args such as prompt_text, prompt_wav_path, and reference_wav_path still use str = None.

Could you please include these related nullable annotations too? That would make the PR consistent and complete. Thanks!