Use CPU-only PyTorch wheels on GitHub Actions

[c-bata]

Motivation

On GitHub Actions environment, PyTorch wheels that contains CUDA-related files are not required. By explicitly opting for the installation of CPU-only wheels, which have smaller file sizes, we can speed up the overall installation process.

Suggestion

Consider adding the --index-url https://download.pytorch.org/whl/cpu option to all pip install commands in GitHub Actions workflow files. It should be noted that some workflows are already utilizing this option, as shown below.

$ git grep torch .github/
.github/workflows/coverage.yml:        pip install --progress-bar off .[optional] --extra-index-url https://download.pytorch.org/whl/cpu
.github/workflows/performance-benchmarks-bayesmark.yml:        pip install --progress-bar off .[benchmark] --extra-index-url https://download.pytorch.org/whl/cpu
.github/workflows/performance-benchmarks-kurobako.yml:        pip install --progress-bar off .[benchmark] --extra-index-url https://download.pytorch.org/whl/cpu
.github/workflows/performance-benchmarks-mo-kurobako.yml:        pip install --progress-bar off .[benchmark] --extra-index-url https://download.pytorch.org/whl/cpu
.github/workflows/speed-benchmarks.yml:        pip install --progress-bar off .[benchmark] --extra-index-url https://download.pytorch.org/whl/cpu
.github/workflows/sphinx-build.yml:        pip install --progress-bar off -U .[document] --extra-index-url https://download.pytorch.org/whl/cpu
.github/workflows/sphinx-build.yml:        pip install --progress-bar off -U .[document] --extra-index-url https://download.pytorch.org/whl/cpu
.github/workflows/tests-storage.yml:        pip install --progress-bar off .[optional] --extra-index-url https://download.pytorch.org/whl/cpu
.github/workflows/tests-with-minimum-versions.yml:        pip install --progress-bar off .[optional] --extra-index-url https://download.pytorch.org/whl/cpu

See https://pytorch.org/get-started/locally/ for details.

Additional context (optional)

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