AMD: enable ROCm torch on gfx906 (MI50 / Radeon VII) on Linux (#7354)

* Add community-maintained legacy support path for gfx906 (MI50 / Radeon VII)

rocm6.4+/7.x torch wheels bundle ROCm libraries whose Tensile kernels
dropped gfx906 (rocBLAS 'TensileLibrary.dat ... not read for gfx906',
ROCm/TheRock#1844), so on MI50/Vega 20 hosts with newer ROCm the
installer picked wheels that fail at the first BLAS call. The rocm6.3
index is the last one whose wheels run on gfx906 (torch 2.7.0 verified
on MI50 32GB, up to 2.9 in community use). Dynamo/Inductor codegen is
also broken on this arch, crashing compiled graphs that train fine in
eager mode.

- install.sh: when the runtime GPU is gfx906 and the picked index is
  newer than rocm6.3, reroute torch to the rocm6.3 index and reset the
  constraint trio to the default <2.11 window (a rocm7.2 pick raises
  the floor to 2.11, which rocm6.3 cannot satisfy), with a legacy-path
  warning.
- install_python_stack.py: mirror the reroute in _ensure_rocm_torch
  using the _default pkg specs, including repairing an existing
  +rocm7.x torch and leaving a working rocm6.3 install alone.
- device_type.py: default TORCHDYNAMO_DISABLE / TORCH_COMPILE_DISABLE /
  UNSLOTH_COMPILE_DISABLE on gfx906 (setdefault, user override wins).

Windows allowlists are untouched: repo.amd.com publishes no gfx906
wheel family (verified in the RDNA2 enablement PR). 16-bit LoRA and
full finetuning work out of the box; 4-bit QLoRA needs a source-built
bitsandbytes for gfx906. Based on the verified MI50 32GB setup in
namnguyen0503/mi50-gfx906-unsloth-bnb4bit-lab.

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

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

* gfx906: second Codex pass (bnb skip under pin, override beats Strix)

- Compute the gfx906 runtime-target flag independently of any torch-index
  pin or Strix override, so the bitsandbytes skip still applies when a user
  pins the ROCm index and sets UNSLOTH_ROCM_GFX_ARCH=gfx906 (the pin
  suppresses the torch reroute, not the bnb skip). Probe only when no pin
  is set (an explicit pin means don't second-guess it, matching the Strix
  path's asserted no-probe invariant); an explicit gfx906 override needs
  no probe.
- Let UNSLOTH_ROCM_GFX_ARCH=gfx906 suppress the Strix reroute (both
  install.sh and install_python_stack.py) so a mixed Strix + MI50 host
  routes to rocm6.3 instead of the gfx1151 wheels probe order would pick.
- Fix test_hardcoded_torch_constraint: the default <2.11 window literal now
  legitimately appears on two TORCH_CONSTRAINT= assignments (default + the
  gfx906 reroute reset after the rocm7.2 floor bump); assert it only ever
  appears on assignment lines, never on a pip install line (its real intent).

New tests: bnb skipped under an explicit pin, gfx906 override wins over
Strix, install.sh suppresses Strix on the override. rocm_support +
selection + cross-platform parity: 667 passed; structural constraint 9/9.

* gfx906: collapse single-line asserts to match pre-commit formatting

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

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

* gfx906: keep bnb skip + rocm6.3 routing correct under pins and suffixed overrides

Address the four Codex P2 findings on #7354:

- bnb skip under a pinned index (install.sh + install_python_stack.py):
  a real gfx906 host that pins UNSLOTH_TORCH_INDEX_URL to rocm6.3 without also
  setting UNSLOTH_ROCM_GFX_ARCH no longer reinstalls the generic bitsandbytes
  wheel over a source-built gfx906 bnb. A pin now suppresses only the torch
  reroute, not the gfx906 detection used for the bnb skip (Python drops the pin
  gate on _runtime_is_gfx906; bash _is_gfx906_bnb_skip probes via
  _probe_amd_gfx_arch when the index is pinned).

- clear the Radeon marketing-name flag for every gfx906 target, not only when
  the >=6.4 reroute fires, so a Radeon VII already on rocm6.3 does not divert to
  the repo.radeon.com branch (whose wheels lack gfx906 kernels).

- normalize a copied HIP gcnArchName (gfx906:sramecc-:xnack- -> gfx906) before
  the exact comparisons in install.sh and install_python_stack.py, mirroring
  device_type.py.

Tests: relax the three Strix-pin tests (the gfx probe may now run for the bnb
flag but must not reroute the pinned index) and add coverage for the pinned
bnb skip, the suffixed override, and the bash Radeon-clear / pinned-probe paths.

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

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

* gfx906: log skipped vLLM aimv2 fix + robust source-scan test bounds

Follow-up review polish:
- import_fixes: log at info level when the vLLM aimv2 fix is skipped because
  the dist metadata is unreadable, so the skip is diagnosable instead of silent.
- test_rocm_support: bound the gfx906 install.sh source-scan on the ';;' that
  closes its case arm via a shared _gfx906_reroute_block helper, replacing the
  brittle fixed-length (3200/3800) slices that shift when the block grows.

* gfx906: trim whitespace on UNSLOTH_ROCM_GFX_ARCH in install.sh (py parity)

The bash gfx906 comparisons lowercased and stripped the gfx906:… feature
suffix but not surrounding whitespace, while the Python paths do .strip().
A stray newline (e.g. export UNSLOTH_ROCM_GFX_ARCH=$(cmd)) would make bash
miss gfx906 while Python catches it. Trim with `tr -d '[:space:]'` at both
comparison sites so the reroute target and bnb-skip agree across bash/Python.

* gfx906: remove generic bitsandbytes pulled in transitively after the skip

---------

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

tests/python/test_tokenizers_and_torch_constraint.py

@@ -88,11 +88,21 @@ class TestStructuralTorchConstraint:
         """$TORCH_CONSTRAINT must appear in a uv pip install line."""
         assert '"$TORCH_CONSTRAINT"' in self._sh
 
-    def test_hardcoded_torch_constraint_only_once(self):
-        """The hard-coded torch>=2.4,<2.11.0 string should appear exactly once
-        in install.sh (the default assignment), not in pip install lines."""
-        count = self._sh.count('"torch>=2.4,<2.11.0"')
-        assert count == 1, f"Expected 1, found {count}"
+    def test_hardcoded_torch_constraint_only_on_assignments(self):
+        """The hard-coded torch>=2.4,<2.11.0 string must only appear on
+        TORCH_CONSTRAINT= assignment lines, never on a pip/uv install line
+        (those must reference $TORCH_CONSTRAINT). Two assignments are expected:
+        the default, and the gfx906 (MI50) reroute that restores the default
+        <2.11 window after the rocm7.2 floor bump raised it to 2.11."""
+        hits = [ln for ln in self._sh.splitlines() if '"torch>=2.4,<2.11.0"' in ln]
+        assert hits, "default constraint literal missing from install.sh"
+        for ln in hits:
+            assert (
+                "TORCH_CONSTRAINT=" in ln
+            ), f"torch>=2.4,<2.11.0 hardcoded off a TORCH_CONSTRAINT= assignment: {ln.strip()!r}"
+            assert (
+                "pip install" not in ln
+            ), f"torch>=2.4,<2.11.0 hardcoded on a pip install line: {ln.strip()!r}"
 
     def test_tightening_guarded_by_skip_torch(self):
         """The block must check SKIP_TORCH=false."""