studio: skip flash-attn install on Blackwell GPUs (sm_100+) (#5420)

* studio: skip flash-attn install on Blackwell GPUs (sm_100+)

Dao-AILab does not publish prebuilt flash-attn wheels for sm_100, sm_120,
or sm_121, and the older-arch wheels fail to load on Blackwell. Add a
shared has_blackwell_gpu() helper and gate both the install-time
(install_python_stack._ensure_flash_attn) and runtime
(worker._ensure_flash_attn_for_long_context) paths on it. Detection uses
nvidia-smi --query-gpu=compute_cap, which works on Linux and Windows.

* test: stub has_blackwell_gpu in pre-existing runtime flash-attn tests

prefers_prebuilt_wheel and falls_back_to_pypi exercise the install
paths that the Blackwell guard now short-circuits. Make them explicit
about non-Blackwell so they pass on real Blackwell hosts.

* studio: cache has_blackwell_gpu, skip Blackwell warning under NO_TORCH

- Wrap has_blackwell_gpu in functools.lru_cache so repeated calls in a
  single process avoid redundant nvidia-smi spawns. Tests clear the
  cache via setup_method/teardown_method.
- In _ensure_flash_attn, run the NO_TORCH short-circuit before the
  Blackwell check so GGUF-only users (who never install torch anyway)
  do not see a Blackwell warning. Blackwell check still runs above the
  IS_WINDOWS / IS_MACOS gates so Blackwell-on-Windows users still see
  the explicit reason rather than a silent OS skip.

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

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

* test: add has_blackwell_gpu to mlx worker test wheel_utils stub

test_mlx_training_worker_config loads worker.py against a hand-rolled
utils.wheel_utils stub. Adding has_blackwell_gpu to the stub symbol
list so worker's import line resolves.

---------

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

studio/install_python_stack.py

@@ -28,6 +28,7 @@ if str(_BACKEND_DIR) not in sys.path:
 from backend.utils.wheel_utils import (
     flash_attn_package_version,
     flash_attn_wheel_url,
+    has_blackwell_gpu,
     install_wheel,
     probe_torch_wheel_env,
     url_exists,
@@ -628,10 +629,19 @@ def _flash_attn_install_disabled() -> bool:
 
 
 def _ensure_flash_attn() -> None:
-    if NO_TORCH or IS_WINDOWS or IS_MACOS:
-        return
     if _flash_attn_install_disabled():
         return
+    if NO_TORCH:
+        return
+    if has_blackwell_gpu():
+        _step(
+            "warning",
+            "Skipping flash-attn: Blackwell GPU detected (sm_100+); no compatible prebuilt wheel",
+            _cyan,
+        )
+        return
+    if IS_WINDOWS or IS_MACOS:
+        return
     if (
         subprocess.run(
             [sys.executable, "-c", "import flash_attn"],