Fix metadata-only setup commands for PyTorch bindings

[ayushozha]

Summary

• skip GPU architecture detection for metadata-only setup.py commands such as egg_info
• replace the deprecated pkg_resources version parsing import with packaging.version.parse
• document TCNN_CUDA_ARCHITECTURES for headless or CPU-only PyTorch install environments

Why

Issue #503 fails during metadata generation before the extension build even starts. That prevents pip from reaching the point where users can provide TCNN_CUDA_ARCHITECTURES explicitly on shared or headless systems.

This change keeps metadata-only commands lightweight, preserves the existing build-time requirement for a known compute capability, and updates the setup script to work in a current setuptools environment where pkg_resources is no longer available.

Validation

• python -m py_compile bindings/torch/setup.py
• python setup.py egg_info
• python setup.py --name
• python setup.py build_ext now fails fast with a clear TCNN_CUDA_ARCHITECTURES message instead of failing during metadata generation

Closes #503.