[testing] move pytest to be inside the function (#4087)

colossalai/testing/pytest_wrapper.py

 """
 This file will not be automatically imported by `colossalai.testing`
-as this file has a dependency on `pytest`. Therefore, you need to 
+as this file has a dependency on `pytest`. Therefore, you need to
 explicitly import this file `from colossalai.testing.pytest_wrapper import <func>`.from
 """
 
-import pytest
 import os
 
 
@@ -30,6 +29,12 @@ def run_on_environment_flag(name: str):
         pytest test_for_something.py
 
     """
+    try:
+        import pytest
+    except ImportError:
+        raise ImportError(
+            'This function requires `pytest` to be installed, please do `pip install pytest` and try again.')
+
     assert isinstance(name, str)
     flag = os.environ.get(name.upper(), '0')