* Definition and type checking for max pool

* Implement kernel, integrate into INTERPRETER, add a few unit tests, make function result type mismatch error message more informative (still need to update tests to reflect that)

* Temporarily delete unit tests to ease merge

* Temporarily delete unit tests to ease merge

* Restore deleted unit tests

* Fix a broken error message check in the unit tests

* Update to handle various TensorViewType-related things going away; add NGVM support

* Add codegen case

* Change various get_blah_shape methods to return const refs, and while we're here, make a similar change where it should have been done in convolution

* Use NDArray for max-pool tests