fix bug in placer holder (#395)

* fix bug in placer holder

* remove redundent comment

Co-authored-by: liukai <your_email@abc.example>

mmrazor/utils/placeholder.py

@@ -1,6 +1,4 @@
 # Copyright (c) OpenMMLab. All rights reserved.
-
-
 def get_placeholder(string: str) -> object:
     """Get placeholder instance which can avoid raising errors when down-stream
     dependency is not installed properly.
@@ -19,13 +17,7 @@ def get_placeholder(string: str) -> object:
         raise ImportError(
             f'`{package_name}` is not installed properly, plz check.')
 
-    class PlaceHolderMetaclass(type):
-        """Used to support usage of PlaceHolder.xxxx."""
-
-        def __getattr__(self, name):
-            raise_import_error(string)
-
-    class PlaceHolder(metaclass=PlaceHolderMetaclass):
+    class PlaceHolder():
 
         def __init__(self) -> None:
             raise_import_error(string)

tests/test_utils/test_placeholder.py

@@ -12,7 +12,7 @@ class TestPlaceholder(unittest.TestCase):
         holder = get_placeholder('test')
         with pytest.raises(ImportError):
             holder()
-        with pytest.raises(ImportError):
-            holder.a
-        with pytest.raises(ImportError):
-            holder.a()
+        from mmrazor.models.architectures.dynamic_ops import DynamicMixin
+
+        class tmp(holder, DynamicMixin):
+            pass