Rename math.py to layer_math.py

* Fix #903

Author: reyoung

python/paddle/trainer_config_helpers/__init__.py

@@ -20,6 +20,4 @@
 from networks import *
 from optimizers import *
 from attrs import *
-
-# This will enable operator overload for LayerOutput
-import math as layer_math
+import layer_math