[MLIR] Add a flag to control ngraph dialect fusion pass. (#4362)
[MLIR] Add a flag to control ngraph dialect fusion pass. (#4362)

* [MLIR] Add a flag to control ngraph dialect fusion pass.

* Address PR feedback.

Co-authored-by: Scott Cyphers <7bb9a997e5f2906b81b1e1515637af43501e2590@users.noreply.github.com>

NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph