[Doc] Update pip install instruction for testing dependencies

[ztang2370]

Update the pip install dev requirements instruction in the doc, following up on #17576.
Running pip install -r requirements/dev.txt will give error:

ERROR: Could not find a version that satisfies the requirement torch==2.7.0+cu128 (from versions: 2.2.0, 2.2.1, 2.2.2, 2.3.0, 2.3.1, 2.4.0, 2.4.1, 2.5.0, 2.5.1, 2.6.0, 2.7.0)
ERROR: No matching distribution found for torch==2.7.0+cu128

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[DarkLight1337]

This only applies for developers using CUDA though. How about other backends?

[ztang2370]

Thanks for pointing this out! However, since torch==2.7.0+cu128 is now specified in requirements/test.txt (updated via #17576 last week), installing it requires access to the PyTorch CUDA 12.8 wheels via pip install -r requirements/dev.txt --extra-index-url https://download.pytorch.org/whl/cu128. I can think of 2 options:

1. Make requirements/test.txt backend-agnostic:
   Change requirements/test.txt back to use a backend-neutral spec:
   torch==2.7.0
   Developers using cuda can then install their desired CUDA variant manually, e.g.:
   pip install torch==2.7.0+cu128 --extra-index-url https://download.pytorch.org/whl/cu128
   Add a note in the README or contributing guide like:

Note: If you're using CUDA and install torch==2.7.0 without a specific build, it will default to the CUDA 12.6 variant. However, CUDA 12.6 builds currently have known issues. It’s recommended to install the CUDA 12.8 build explicitly. See PyTorch installation guide.

2. Keep torch==2.7.0+cu128, but document clearly:
   If we keep the CUDA-specific spec in requirements/test.txt, we should update the doc to say:
   pip install -r requirements/dev.txt --extra-index-url https://download.pytorch.org/whl/cu128
   And add a note:

Note: This assumes you're using CUDA 12.8. For other environments (CPU-only, different CUDA versions, ROCm), you may need to adjust the torch installation manually.

Let me know which direction you'd prefer. I'm happy to update the PR accordingly.

[DarkLight1337]

cc @houseroad @mgoin

[ztang2370]

Updated by adding comment

[ztang2370]

Hi there, I added the comment to make it clearer.
@DarkLight1337 @houseroad @mgoin

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ztang2370.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork