*  Use Eigen kernel for ScatterAdd.

* Emit ScatterAdd Eigen kernel for CODEGEN.

Add comments.

Address PR feedback.

* Add more unit tests.

Fix style error.

Add ScatterAdd to AssignOpMap.

* Combine non-scalar cases together.

* Address PR feedback.

* Fix a bug.

* Use reshape to make the shapes of two slices match.

* Rename variables.

Fix bugs.

Use helper function.

Add one unit test.

* Add reshape back.