Use functools.cache on has_efa (#163439)

Cache the result of `has_efa` by `functools.cache`.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/163439
Approved by: https://github.com/janeyx99

torch/testing/_internal/common_distributed.py

@@ -1,6 +1,7 @@
 # mypy: ignore-errors
 
 import faulthandler
+import functools
 import itertools
 import logging
 import multiprocessing
@@ -1140,30 +1141,24 @@ def run_subtests(
         c10d.barrier()
 
 
-# Cannot use functools.cache as it requires python 3.9
-EFA_PROBE_RESULT = None
-
-
+@functools.cache
 def has_efa() -> bool:
     """
     If shell command `fi_info -p efa -t FI_EP_RDM` returns exit code 0 then we assume that the machine has
     Libfabric EFA interfaces and EFA software components installed,
     see https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/efa-start.html.
     """
-    global EFA_PROBE_RESULT
-    if EFA_PROBE_RESULT is not None:
-        return EFA_PROBE_RESULT
 
     try:
-        EFA_PROBE_RESULT = (
+        return (
             subprocess.run(
                 ["fi_info", "-p", "efa", "-t", "FI_EP_RDM"], check=False
             ).returncode
             == 0
         )
     except FileNotFoundError:
-        EFA_PROBE_RESULT = False
-    return EFA_PROBE_RESULT
+        pass
+    return False
 
 
 def tp_transports():