Created by: baojun-nervana

This PR is to move ngraph files from /framework to operators/ngraph. This is part of effort to split the PR https://github.com/PaddlePaddle/Paddle/pull/14948

• ngraph_bridge.* was moved into operators/ngraph
• ngraph_operator.* was moved as well
  • NgraphEngine was taken out of ngraph_operator.* as separate file. (This is needed by PR #14948)