[BE] Refactor functions from optional_submodules (#155954)

And use `pathlib.Path` instead of `os.path`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/155954
Approved by: https://github.com/Skylion007
ghstack dependencies: #155947

tools/optional_submodules.py

@@ -1,43 +1,46 @@
 import os
-import subprocess
 from pathlib import Path
+from subprocess import check_call
 
 
 repo_root = Path(__file__).absolute().parent.parent
-third_party_path = os.path.join(repo_root, "third_party")
+third_party_path = repo_root / "third_party"
+
+
+def _read_file(path: Path) -> str:
+    with path.open(encoding="utf-8") as f:
+        return f.read().strip()
+
+
+def _checkout_by_tag(repo: str, tag: str) -> None:
+    check_call(
+        [
+            "git",
+            "clone",
+            "--depth",
+            "1",
+            "--branch",
+            tag,
+            repo,
+        ],
+        cwd=third_party_path,
+    )
 
 
 def read_nccl_pin() -> str:
     nccl_file = "nccl-cu12.txt"
-    if os.getenv("DESIRED_CUDA", "").startswith("11") or os.getenv(
-        "CUDA_VERSION", ""
-    ).startswith("11"):
+    if os.getenv("DESIRED_CUDA", os.getenv("CUDA_VERSION", "")).startswith("11"):
         nccl_file = "nccl-cu11.txt"
-    nccl_pin_path = os.path.join(
-        repo_root, ".ci", "docker", "ci_commit_pins", nccl_file
-    )
-    with open(nccl_pin_path) as f:
-        return f.read().strip()
+    nccl_pin_path = repo_root / ".ci" / "docker" / "ci_commit_pins" / nccl_file
+    return _read_file(nccl_pin_path)
 
 
 def checkout_nccl() -> None:
     release_tag = read_nccl_pin()
     print(f"-- Checkout nccl release tag: {release_tag}")
-    nccl_basedir = os.path.join(third_party_path, "nccl")
-    if not os.path.exists(nccl_basedir):
-        subprocess.check_call(
-            [
-                "git",
-                "clone",
-                "--depth",
-                "1",
-                "--branch",
-                release_tag,
-                "https://github.com/NVIDIA/nccl.git",
-                "nccl",
-            ],
-            cwd=third_party_path,
-        )
+    nccl_basedir = third_party_path / "nccl"
+    if not nccl_basedir.exists():
+        _checkout_by_tag("https://github.com/NVIDIA/nccl", release_tag)
 
 
 if __name__ == "__main__":