Created by: qingqing01

Fix https://github.com/PaddlePaddle/Paddle/issues/6841

• Time: collected by yep + pprof based the config of sentiment analysis

  • forward: LSTMUnitFunctor: 9.52s -> 7.95s
  • backward: LSTMGradUnitFunctor: 5.60s -> 3.68s

• Refine the code style for the activation type

  • But the old code is not removed, since it is used in GRU operator. This will be fixed in next PR.

• Still use std::string type in the attribute definition.

  • By using the int type in the attribute definition, the time is close with the std::string type.