Add configurable PyTorch mirror via UNSLOTH_PYTORCH_MIRROR env var (#5024)

* Add configurable PyTorch mirror via UNSLOTH_PYTORCH_MIRROR env var

When set, UNSLOTH_PYTORCH_MIRROR overrides the default
https://download.pytorch.org/whl base URL in all four install scripts
(install.sh, install.ps1, studio/setup.ps1, studio/install_python_stack.py).
When unset or empty, the official URL is used. This lets users behind
corporate proxies or in regions with poor connectivity to pytorch.org
point at a local mirror without patching scripts.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add pytest for UNSLOTH_PYTORCH_MIRROR in install_python_stack.py

Tests that _PYTORCH_WHL_BASE picks up the env var when set, falls back
to the official URL when unset or empty, and preserves the value as-is
(including trailing slashes).

* Remove stale test assertions for missing install.sh messages

* Fix GPU mocking in test_get_torch_index_url.sh

Extract _has_usable_nvidia_gpu and _has_amd_rocm_gpu alongside
get_torch_index_url so the GPU-presence checks work in tests.
Add -L flag handling to mock nvidia-smi so it passes the GPU listing
check. All 26 tests now pass on CPU-only machines.

* Strip trailing slash from UNSLOTH_PYTORCH_MIRROR to avoid double-slash URLs

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

studio/install_python_stack.py

@@ -49,7 +49,9 @@ _ROCM_TORCH_INDEX: dict[tuple[int, int], str] = {
     (6, 1): "rocm6.1",
     (6, 0): "rocm6.0",
 }
-_PYTORCH_WHL_BASE = "https://download.pytorch.org/whl"
+_PYTORCH_WHL_BASE = (
+    os.environ.get("UNSLOTH_PYTORCH_MIRROR") or "https://download.pytorch.org/whl"
+).rstrip("/")
 
 # bitsandbytes continuous-release_main wheels with the ROCm 4-bit GEMV fix
 # (bnb PR #1887, post-0.49.2). bnb <= 0.49.2 NaNs at decode shape on every