Fix auto-resume training from checkpoint (huggingface#9822)

* Fix auto-resume training from checkpoint

* style fixes

src/transformers/trainer_utils.py

@@ -77,15 +77,19 @@ class TrainOutput(NamedTuple):
 
 
 PREFIX_CHECKPOINT_DIR = "checkpoint"
-_re_checkpoint = re.compile(r"^" + PREFIX_CHECKPOINT_DIR + r"\-(\d)+$")
+_re_checkpoint = re.compile(r"^" + PREFIX_CHECKPOINT_DIR + r"\-(\d+)$")
 
 
 def get_last_checkpoint(folder):
     content = os.listdir(folder)
-    checkpoints = [path for path in content if _re_checkpoint.search(path) is not None and os.path.isdir(path)]
+    checkpoints = [
+        path
+        for path in content
+        if _re_checkpoint.search(path) is not None and os.path.isdir(os.path.join(folder, path))
+    ]
     if len(checkpoints) == 0:
         return
-    return max(checkpoints, key=lambda x: int(_re_checkpoint.search(x).groups()[0]))
+    return os.path.join(folder, max(checkpoints, key=lambda x: int(_re_checkpoint.search(x).groups()[0])))
 
 
 class EvaluationStrategy(ExplicitEnum):