* add inner function _elementwise_op_with_axis

* fix transformer_model

* polish API code

* remove elementwise_div/mul api

* delete API in __all__

* delete elementwise_mul completely

* polish elementwise_mul call

* polish internal api

* resolve conflict, fix rnn.py

* use non-inplace call

* delete elementwise_mul api test

* delete elementwise_mul api test

* clean elementwise_add/sub

* restore _elementwise_op_in_dygraph in nn.py