* Add conv2d max_pool backward op

* Added tests

* Fix testing

* Address comments

* Change dot to matmul

* Address comments

* Break down indicator function

* Make greater, less numpy compatible

* Add creating gradient symbol

* Fix lint

* Address comments

* Fix typo

* Address comment

* add additional field into node for autograd

* check inputs reachable

* fix

* Handling duplicate NodeEntries on the edge of the graph

* Fix docs and segfault

* Suggestions from review

* Added attr_parser check

* Change TShape::index_t to int64_t

* Add comment

* Make Tuple::Save&Load dtype generic

* trigger update

* Fix lint

* Fix comment

* Change index_t to dim_t

* Remove legacy index_t

* change to match by both op and name

* add zero prop for non differentiable ops

* Update infer_shape_type.cc

* Fix warnings from Logging, enable Plan memory to take external memory

* fix external memory id

* fix graph

* Enable optional dependency memory and attr hint

* fix travis

* [SYMBOL] Change list_input->list_input_names, add list_input_variables

* fix