Add note when import core.so error (#11918)

* add note when import core.so error

* update

python/paddle/fluid/framework.py

@@ -19,7 +19,16 @@ import re
 import numpy as np
 
 import proto.framework_pb2 as framework_pb2
+try:
     from . import core
+except ImportError, e:
+    raise ImportError(
+        """NOTE: You may need to run \"export LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH\"
+    if you encounters \"libmkldnn.so not found\" errors. If you have python
+    installed in other directory, replace \"/usr/local/lib\" with your own
+    directory. The original error is: """ % str(e))
+except Exception, e:
+    raise e
 import unique_name
 
 __all__ = [