test=develop, fix memory leak in dygraph (#18998)

python/paddle/fluid/framework.py

@@ -272,11 +272,15 @@ def name_scope(prefix=None):
               g = f - 1
     """
     # TODO(panyx0718): Only [0-9a-z].
+    # in dygraph we don't need namescope since it will cause mem leak
+    if not in_dygraph_mode():
         assert prefix, "namescope prefix cannot be empty."
         global _name_scope
         _name_scope = _name_scope.child(prefix)
         yield
         _name_scope = _name_scope.parent()
+    else:
+        yield
 
 
 def _full_name_scope():