moved ensure_path_exists to utils.common (#591)

colossalai/utils/common.py

 #!/usr/bin/env python
 # -*- encoding: utf-8 -*-
+import os
 import random
 import socket
+from pathlib import Path
 from typing import List, Union
 
 import torch
@@ -39,6 +41,13 @@ def print_rank_0(msg: str, logger=None):
             logger.info(msg)
 
 
+def ensure_path_exists(filename: str):
+    # ensure the path exists
+    dirpath = os.path.dirname(filename)
+    if not os.path.exists(dirpath):
+        Path(dirpath).mkdir(parents=True, exist_ok=True)
+
+
 def free_port():
     while True:
         try:
@@ -103,7 +112,6 @@ def conditional_context(context_manager, enable=True):
 
 
 class model_branch_context(object):
-
     def __enter__(self):
         self.env_status = env.save()
 
@@ -123,7 +131,7 @@ def _calc_l2_norm(grads):
             colossal_C.multi_tensor_l2norm,
             dummy_overflow_buf,
             [grads],
-            False    # no per-parameter norm
+            False  # no per-parameter norm
         )
     return norm