Fix bad import name in trainer_config_helpers.

* math => layer_math
* Fix #903

python/paddle/trainer_config_helpers/__init__.py

@@ -22,4 +22,4 @@ from optimizers import *
 from attrs import *
 
 # This will enable operator overload for LayerOutput
-import math
+import math as layer_math

python/paddle/trainer_config_helpers/tests/configs/math_ops.py

 from paddle.trainer_config_helpers import *
-from paddle.trainer_config_helpers import math
 
 settings(batch_size=1000, learning_rate=1e-5)
 
 x = data_layer(name='data', size=100)
-x = math.exp(x)
-x = math.log(x)
-x = math.abs(x)
-x = math.sigmoid(x)
-x = math.square(x)
-x = math.square(x)
+x = layer_math.exp(x)
+x = layer_math.log(x)
+x = layer_math.abs(x)
+x = layer_math.sigmoid(x)
+x = layer_math.square(x)
+x = layer_math.square(x)
 y = 1 + x
 y = y + 1
 y = x + y