Update model loading to support local checkpoints

[reisen669]

• Modified hy3dgen.shapegen.utils.smart_load_model to prioritize loading models from a direct local path if it exists, before checking the cache or Hugging Face Hub.
• Updated minimal_demo.py to use './Hunyuan3D-2' as the model path, enabling it to run with locally downloaded checkpoints in that directory structure.

[tencent-adm]

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