To solve this, we maintain the list of DefaultNameFactory used in by trainer_config_helper,
and reset the state at the beginning of each parse_config call.

Change-Id: I13c7574dc8f0b6bc6f6b7c92eb425e2c52c926e8

* Add elementwise math operations
This allows use to use expressions like: y=log(1+exp(x))
Also added unittests for ActivationFunction
* Enforce keyword arguments for non-positional arguments
* Add LogActivation to doc

ISSUE=4586495

git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1408 1ad973e4-5ce8-4261-8a94-b56d1f490c56