Created by: baojun-nervana

This PR is to enable a paddle-like ngraph_engine operator. The method is to scan ProgramDesc and replace ngraph supported ops as a ngraph_engine op.

The ngraph_engine op has an attribute "graph" holding a string of serialized blockdesc, and the attribute "interval" defining the op indices to pass to ngraph_op. As an example,

interval = [4, 6] : op 4 and 5 will be combined as a ngraph_engine op.

In the executor.cc, the trigger is still controlled by FLAGS_use_ngraph. It will call operators::NgraphEngine::EnableNgraph(pdesc) to update the pdesc. The ngraph_op engine will substitute the supported op and ngraph_op is registered and can be created through OpRegistry::CreateOp(*op_desc).

The code is located under operators/ngraph. The NgraphEngine op is mostly moved from framework/ngraph_operactor.. After this PR is approved. ngraph_operator. can be deleted and ngraph_bridge.* can be moved to oeprator/ngraph as well.