Add gpu install extra and document CPU/GPU install paths

[ParamThakkar123]

Motivation

• Provide a simple opt-in way to install CUDA-enabled PyTorch wheels when a GPU is available via pip install torchwm[gpu] while keeping pip install torchwm CPU-first for portable installs.
• Make project metadata and package index routing compatible with uv so the CUDA PyTorch index is only used when the gpu extra is requested.

Description

• Add a gpu optional-dependency to pyproject.toml under [project.optional-dependencies] and to extras_require in setup.py that includes torch, torchvision, and torchaudio.
• Update uv configuration in pyproject.toml ([tool.uv] / [tool.uv.sources] and [[tool.uv.index]]) so torch, torchvision, and torchaudio are routed to the CUDA 12.1 PyTorch index when the gpu extra is requested and are not forced to the CUDA index by default.
• Update documentation: README.md, docs/source/installation.md, and docs/source/getting_started.md now document pip install torchwm (CPU/default) and pip install torchwm[gpu] (CUDA-enabled) with guidance about uv behavior.
• Update the project lock/metadata (uv.lock entries) so the new gpu extra is reflected and package metadata include extra == 'gpu' markers for the PyTorch packages.

Testing

• Verified pyproject.toml parsing and presence of the gpu extra with a small tomllib validation script (passed).
• Performed an editable-install dry-run with python -m pip install -e . --dry-run --no-deps --no-build-isolation which reported the package would install (passed).
• Ran python setup.py egg_info which completed and generated metadata (passed).
• Built a source distribution with uv build --sdist --no-build-isolation which succeeded and produced dist/torchwm-*.tar.gz (passed).
• Ran uv lock --offline --dry-run which failed due to the offline lock cache missing ale-py for the Python >=3.14 resolution split; this failure is an offline cache/network issue and not caused by the gpu extra change (warning/failure unrelated to metadata changes).

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Codex Task

[github-actions]

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