amd: fall back to the PyPI bitsandbytes floor on Windows ROCm too

studio/install_python_stack.py

@@ -1189,7 +1189,7 @@ _rocm_windows_torch_installed: bool = False
 
 
 def _install_bnb_windows_rocm() -> bool:
-    """Install the AMD Windows BNB prerelease wheel. Returns True on success.
+    """Install AMD Windows BNB, pre-release wheel first. Returns True on success.
 
     The continuous-release wheel is intentionally mismatched: the filename
     encodes 1.33.7.preview (parsed as 1.33.7rc0 by PEP 440) while the wheel
@@ -1199,19 +1199,39 @@ def _install_bnb_windows_rocm() -> bool:
     (https://unsloth.ai/docs/get-started/install/amd/amd-hackathon) the wheel
     must be installed with plain pip, not uv, so we force pip (force_pip=True);
     plain pip performs no wheel filename/metadata check.
+
+    When that URL is blocked, fall back to PyPI. Its win_amd64 wheel ships
+    libbitsandbytes_rocm{714,72}.dll from 0.50.0 on, so the fallback is a real
+    ROCm build; before 0.50.0 it was CUDA-only, which is why there was none.
     """
     _bnb_win_url = _BNB_ROCM_PRERELEASE_URLS.get("win_amd64")
-    if _bnb_win_url is None:
-        return False
-    _ok = pip_install_try(
-        "bitsandbytes (AMD Windows, pre-release main)",
-        "--force-reinstall",
-        "--no-cache-dir",
-        "--no-deps",
-        _bnb_win_url,
-        constrain = False,
-        force_pip = True,
-    )
+    _ok = False
+    if _bnb_win_url is not None:
+        _ok = pip_install_try(
+            "bitsandbytes (AMD Windows, pre-release main)",
+            "--force-reinstall",
+            "--no-cache-dir",
+            "--no-deps",
+            _bnb_win_url,
+            constrain = False,
+            force_pip = True,
+        )
+        if not _ok:
+            print(
+                _red(
+                    "   bnb pre-release install failed; falling back to PyPI "
+                    f"{_BNB_ROCM_PYPI_FALLBACK}, which carries the ROCm 4-bit fix"
+                )
+            )
+    if not _ok:
+        _ok = pip_install_try(
+            "bitsandbytes (AMD Windows)",
+            "--force-reinstall",
+            "--no-cache-dir",
+            "--no-deps",
+            _BNB_ROCM_PYPI_FALLBACK,
+            constrain = False,
+        )
     if not _ok:
         return False
     # Detect the actual ROCm DLL suffix in the wheel and set BNB_ROCM_VERSION so bnb
@@ -1701,8 +1721,8 @@ def _ensure_rocm_torch() -> None:
             pass
         if _torch_ok:
             _rocm_windows_torch_installed = True
-            # ROCm torch is already installed, but the AMD Windows BNB wheel is still
-            # needed (the PyPI bitsandbytes ships only CUDA DLLs, fails on ROCm).
+            # ROCm torch is already installed, but bnb still needs the ROCm build
+            # (pre-release wheel, else PyPI >=0.50.0).
             _install_bnb_windows_rocm()
             return
         # torch was wiped between runs; fall through to the full install path
@@ -1780,12 +1800,12 @@ def _ensure_rocm_torch() -> None:
         # separate dependency -- a BNB install failure must NOT roll back the
         # torch ROCm install.
         _rocm_windows_torch_installed = True
-        # Always install AMD Windows bitsandbytes -- the PyPI wheel ships only
-        # CUDA DLLs and fails on ROCm. Install even when torch was already a
-        # ROCm build so `studio update` repairs a broken bnb.
+        # Always install AMD Windows bitsandbytes, even when torch was already a
+        # ROCm build, so `studio update` repairs a broken bnb.
         if not _install_bnb_windows_rocm():
             print(
-                "   Warning: AMD Windows bitsandbytes install failed; "
+                "   Warning: AMD Windows bitsandbytes install failed "
+                "(pre-release and PyPI); "
                 "ROCm torch is installed but bitsandbytes may need manual install"
             )
         return