fix(scripts): pip-install clang-format if missing at codegen time

CI containers running with `--no-build-isolation` (metax, moore,
cambricon) skip `[build-system].requires` and never install
`clang-format` from PyPI; system packages do not provide it
either, so cmake-time codegen fails with `FileNotFoundError`.

Probe `PATH` for `clang-format` at codegen entry; if missing,
`pip install clang-format` into the running interpreter and reuse
the installed binary.  Adds at most a couple of seconds to a
first-time configure on hosts without the binary.

Author: voltjia

scripts/generate_torch_ops.py

@@ -824,14 +824,42 @@ class Operator<{pascal}, kDev, {slot}> : public {pascal} {{
 """
 
 
+def _ensure_clang_format() -> str:
+    """Return the path to a `clang-format` binary, installing the
+    `clang-format` PyPI wheel into the running interpreter if the system
+    does not provide one (CI containers running with
+    `--no-build-isolation` skip `[build-system].requires`)."""
+
+    found = shutil.which("clang-format")
+
+    if found:
+        return found
+
+    print(
+        "`clang-format` not found on PATH; installing `clang-format` from PyPI...",
+        file=sys.stderr,
+    )
+    subprocess.run(
+        [sys.executable, "-m", "pip", "install", "--quiet", "clang-format"],
+        check=True,
+    )
+
+    found = shutil.which("clang-format")
+
+    if not found:
+        raise RuntimeError("`clang-format` still not available after `pip install`.")
+
+    return found
+
+
 def _clang_format(text: str, path: pathlib.Path) -> str:
     """Pipe `text` through `clang-format` so generated headers / sources
     satisfy the same style check (`clang-format` v21) that CI runs.
     `path` informs include sorting (the file's own header should come
     first in a `.cc`)."""
 
     return subprocess.run(
-        ["clang-format", f"--assume-filename={path}"],
+        [_CLANG_FORMAT, f"--assume-filename={path}"],
         input=text,
         capture_output=True,
         text=True,
@@ -868,6 +896,9 @@ def main() -> int:
     )
     args = parser.parse_args()
 
+    global _CLANG_FORMAT
+    _CLANG_FORMAT = _ensure_clang_format()
+
     op_names = args.ops or yaml.safe_load(_OPS_YAML_PATH.read_text())
     aten_entries = yaml.safe_load(_load_aten_yaml())