Separate trainer and updater

[helinwang]

Before we are planning to use a single concept trainer to combine trainer and updater.
E.g.,

paddle.trainer.Momentum
paddle.trainer.MomentumParallel
paddle.trainer.AdagradParallel
paddle.trainer.Adagrad
paddle.trainer.AdagradParallel
paddle.trainer.Adam
paddle.trainer.AdamParallel

The number-of-class will be number-of-trainer x number-of-update-algorithm.

To make number of class fewer, maybe we can separate trainer and updater. And update is a C++ interface with a pure virtual function update:

paddle.trainer.SGD
paddle.trainer.SGDParallel
paddle.updater.Adam
paddle.updater.Adagrad
paddle.updater.Momentum

[wangkuiyi]

I agree. And I am adding this point into the API design.