[WIP] Executor RNN support

[tonyyang-svail]

Overview

Executor

1. Instantiate all the ops in the RNN block
2. Set ops and vars in RNNOp

RNN

1. Replace NetOp to vector<Op>. This will results compilation failure in backward.cc
2. Add vector<string> vars_ to represent temporary variable name inside RNN

Checklist:

• Pass Forward
• Pass Backward
• Clean up

[tonyyang-svail]

This has been done at #5224