Add load_legacy_checkpoint function

tests/tests_pytorch/checkpointing/test_legacy_checkpoints.py

@@ -14,7 +14,6 @@
 import glob
 import os
 import sys
-import threading
 from unittest.mock import patch
 
 import pytest
@@ -80,8 +79,32 @@ def load_model():
         from lightning.pytorch.utilities.migration import pl_legacy_patch
 
         with pl_legacy_patch():
-            _ = torch.load(path_ckpt)    
-
+            _ = torch.load(path_ckpt)
+
+    with patch("sys.path", [PATH_LEGACY] + sys.path):
+        t1 = ThreadExceptionHandler(target=load_model)
+        t2 = ThreadExceptionHandler(target=load_model)
+
+        t1.start()
+        t2.start()
+
+        t1.join()
+        t2.join()
+
+
+@pytest.mark.parametrize("pl_version", LEGACY_BACK_COMPATIBLE_PL_VERSIONS)
+@RunIf(sklearn=True)
+def test_load_legacy_checkpoint_threading(tmpdir, pl_version: str):
+    PATH_LEGACY = os.path.join(LEGACY_CHECKPOINTS_PATH, pl_version)
+    path_ckpts = sorted(glob.glob(os.path.join(PATH_LEGACY, f"*{CHECKPOINT_EXTENSION}")))
+    assert path_ckpts, f'No checkpoints found in folder "{PATH_LEGACY}"'
+    path_ckpt = path_ckpts[-1]
+
+    def load_model():
+        from lightning.pytorch.utilities.migration.utils import load_legacy_checkpoint
+
+        _ = load_legacy_checkpoint(path_ckpt)
+
     with patch("sys.path", [PATH_LEGACY] + sys.path):
         t1 = ThreadExceptionHandler(target=load_model)
         t2 = ThreadExceptionHandler(target=load_model)