* Enable less_than ngraph operator

test=develop

* Added compare unit-tests test=develop

* Update: date && removed import test=develop

test=develop

* add depthwise_conv2d test=develop

* use cpu for ngraph test=develop

This reverts commit f34830e2.

* Enable sqrt operator for the nGraph Bridge.

test=develop

* Update activation_op.h

test=develop

test=develop

*  Enable assign operator for a ngraph, test=develop

* Cross_entropy operators needs to be updated

test=develop

* add increment op to ngraph engine

test=develop

* fix style errors

test=develop

* added lrn op test=develop

* Added CreateConstant method test=develop

* avoid duplicates test=develop

resolve #17147
test=develop

* Add softmax_with_cross_entropy_op test=develop

* simplify implementation test=develop

* take care edge cases test=develop

* use pragma test=develop

test=develop

test=develop

* remove concast_cast and refactor code test=develop

* reduce flag use test=develop

test=develop