add sparse gradients support for RMSPropOptimizier (#2564)

* add sparse gradients support for RMSPropOptimizier

* change as advised

* format code and add note to doc

* add note to api doc

* change as advised

* format code and add note to doc

* solve conflict

* remove modification in .md and duplication in training_ops.cc

* recover code

* remove modification on .md files