User can provide an initializer in ParamAttr. The initializer will be called when at parameters.create().

print_layer is used to print the values of a layer when evaluating the model for debugging purpose. No layer depends on print_layer. It should be added to the topology if it is used to print some layer already in the topology.

Some time we simply want to have a fixed initialization for a model.

correct a typo

optimize code

fix a bug

* Compose GRU step naive layer in trainer config helpers.
  * It is uses mixed_layer for gate.
  * It supports ERROR_CLIPPING, DROPOUT
* Add error clipping in MT demo.
* Fix #1143
* Fix #1891