[testing] ensure concurrent pytest workers use a unique port for torch.dist (#12166)

* ensure concurrent pytest workers use a unique port for torch.distributed.launch

* reword

src/transformers/testing_utils.py

@@ -1249,6 +1249,28 @@ def execute_subprocess_async(cmd, env=None, stdin=None, timeout=180, quiet=False
     return result
 
 
+def pytest_xdist_worker_id():
+    """
+    Returns an int value of worker's numerical id under ``pytest-xdist``'s concurrent workers ``pytest -n N`` regime,
+    or 0 if ``-n 1`` or ``pytest-xdist`` isn't being used.
+    """
+    worker = os.environ.get("PYTEST_XDIST_WORKER", "gw0")
+    worker = re.sub(r"^gw", "", worker, 0, re.M)
+    return int(worker)
+
+
+def get_torch_dist_unique_port():
+    """
+    Returns a port number that can be fed to ``torch.distributed.launch``'s ``--master_port`` argument.
+
+    Under ``pytest-xdist`` it adds a delta number based on a worker id so that concurrent tests don't try to use the
+    same port at once.
+    """
+    port = 29500
+    uniq_delta = pytest_xdist_worker_id()
+    return port + uniq_delta
+
+
 def nested_simplify(obj, decimals=3):
     """
     Simplifies an object by rounding float numbers, and downcasting tensors/numpy arrays to get simple equality test