* LayerNorm (#3630)

* Constructors.

Type prop.

Decompose LayerNorm.

Add serialize.

* Add dummy test case.

* Add dummy type prop test.

* Fix some build errors.

* Remove build errors.

* Update decompose for bprop.

* Change begin_norm_axis default value to 1.

* Style.

* Reorder class members.

* Add actual type prop tests.

* Add fprop test.

* Working on bprop test.

* Bprop tests.

* Allow flattened scale and bias.

* Add support for flattened scale and bias.

* Fix incorrect type_name.

* PlaidML: Decompose fused_op LayerNorm

* Update Backprop constructors.

* PlaidML: Add missing header file.

* Remove doc about removed param.

* Fix type prop tests.

* PlaidML: Disable unit test.

* Fix stats flattening axes bug.

* Upgrade description to type_info.