Improve modern packaging compatibility for local source builds

[han-xudong]

Fixes #2036

Summary

This PR improves PyTorch3D compatibility with modern Python packaging workflows.

Changes

• add a minimal pyproject.toml using setuptools.build_meta
• declare core static project metadata in pyproject.toml
• declare authors, optional-dependencies, and scripts as dynamic
• change extension source paths in setup.py from absolute paths to paths relative to the project root

Why

Modern build frontends are stricter than legacy setup.py install flows.

In particular:

• setuptools local/editable builds reject absolute extension source paths
• modern frontends work better when minimal project metadata is available through pyproject.toml

These changes improve packaging compatibility without changing the runtime API.

Validation

Validated with a local source install through uv using a matching PyTorch / CUDA combination:

tmpdir=$(mktemp -d)
uv venv "$tmpdir/venv"

UV_HTTP_TIMEOUT=300 uv pip install --python "$tmpdir/venv/bin/python" \
  --index-strategy unsafe-best-match \
  setuptools wheel numpy iopath torch==2.10.0+cu128 \
  --index-url https://pypi.org/simple \
  --extra-index-url https://download.pytorch.org/whl/cu128

UV_HTTP_TIMEOUT=300 CUDA_HOME=/usr/local/cuda-12.8 \
PATH="/usr/local/cuda-12.8/bin:$PATH" \
uv pip install --python "$tmpdir/venv/bin/python" \
  /path/to/pytorch3d \
  --no-build-isolation

This completed successfully and built pytorch3d from local source.