* [MLIR] Use call back for MatMul.

* Use callback for Gemm.

* Use mkldnn callback for Softmax.

* Address PR feedback.

* Fix merge errors.

* Change to tail allocation struct.

* Use mkldnn callback for AvgPool.

* Add callbacks for AvgPoolBackprop, MaxPool, and MaxPoolBackprop.

* Fix merge errors.

* Use UnrankedMemRefType for callbacks.

* Address PR feedback.

* Cleanup.

* Address PR feedback.

* Fix a bug.

* Use global variable to hold attributes.

* Convert layout if needed for pooling.

* Address PR feedback.

* Add header.

* Address PR feedback.

* Update Copyright to 2017-2020.

* Address PR feedback.
Co-authored-by: Scott Cyphers <diyessi@users.noreply.github.com>