Use CPU-only PyTorch index to speed up dev installation

[adamtheturtle]

Summary

• vws-python-mock (a dev dependency) transitively requires torch and torchvision
• By default, uv resolves these from PyPI which provides CUDA-enabled wheels on Linux (~916 MB for torch alone)
• This PR follows the faster installation guide to redirect torch and torchvision to the CPU-only PyTorch index

Does it actually make installation faster?

Yes — tested empirically (in VWS-Python/vws-python#2857):

Package	CUDA wheel (PyPI)	CPU-only wheel	Saving
torch (Linux, cp313)	916 MB	189 MB	727 MB
torchvision (Linux, cp313)	8 MB	2 MB	6 MB

~733 MB total reduction per cold-cache install on Linux CI runners.

Implementation notes

[tool.uv.sources] only applies to direct dependencies (not transitive ones), so torch and torchvision must be listed explicitly in optional-dependencies.dev for the source override to take effect. A deptry ignore is added since these packages are not directly imported in the vws-auth-tools source code.

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Note

Low Risk
Dependency/config-only change affecting developer environment installs; no runtime or security-sensitive logic is modified.

Overview
Speeds up dev installs by explicitly adding torch/torchvision to optional-dependencies.dev and configuring uv to resolve them from the CPU-only PyTorch index ([tool.uv]), avoiding large CUDA wheels from PyPI.

Adds a deptry ignore (DEP002) for these newly-explicit dev deps since they’re not directly imported by vws-auth-tools.

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