Address review feedback on the override added in this PR so a pinned index is
honoured everywhere, not just in get_torch_index_url:
- Skip the WSL ROCm bootstrap (root privilege + large downloads, probes
/dev/dxg) when UNSLOTH_TORCH_INDEX_URL / _FAMILY is set; it previously ran
before the override was consulted.
- Skip the Radeon/Strix rerouting (which re-probes the GPU and overwrites the
resolved URL with repo.radeon.com / repo.amd.com) when the index is pinned, so
an explicit ROCm override (e.g. UNSLOTH_TORCH_INDEX_FAMILY=rocm6.4) is kept.
- install_python_stack.py: derive _TORCH_BACKEND from the override when
UNSLOTH_TORCH_BACKEND is unset (standalone studio update), so _ensure_rocm_torch
/ _ensure_cuda_torch repair to the requested family instead of re-detecting.
- Strip ALL leading/trailing slashes in the shell override to match the Python
side (avoids 404s on strict pip proxies).
Adds test cases for double-slash and leading/trailing-slash overrides.
b02a609af5