Refine optimizers (#1407)
* Refine optimizers
* Address PR comments
* Changes from PR comments and discussion.
* Fixed signed/unsigned mismatch
* Address PR comments
* Address PR comments
* Fix linux build
* Fix issue with mkldnn logic.
* Turn off optimizers by default for operator unit tests.
* Handle edge case of graph with no nodes in partitioner so all execution providers don't need to.
* Comment out change to turn off optimizers for unit tests. Add details on what needs to be done to re-enable.
