This patch reverts one of my earlier patches (squashed in #2710) to
reduce bandwidth requirements of git clone, in this particular case we
are checking out a specific hash rather than a tag or branch name. The
--branch option to git clone permits tags or branches but does not
permit a specific hash.

Code like this can't be built with NV OpenCL, and it needs an explicit type
  converison for ternary expression if return type is uchar.

       uchar i = 0, j = 0;
       uchar t = max((uchar)j, ((i > 0) ? (uchar)1 : (uchar)0));

* adding _contrib_BilinearResize2D op from mxnet

* error fixed

* use resize instead of upsample

git clone --branch=xxx won't take a hash, switch from the hash to the
tag that represents that hash.

* Add shapeof op in topi

* Add relay shape_of op

* Add constant folding for shape_of

* Allow shape op to specify dtype

* Add mxnet converter for shape_array

* lint

* lint

* Add doc

* start adding reverse

* reverse updated

* reverse uses topi::flip

* typo fixed

* comment addressed

* exp simplified

* initial commit

* add python frontend and module tests

* add unit tests for function pass and optimize interface

* add ExprPass

* remove PassState and pass context for run

* add required_passes

* return module

* remove move

* fix minor reviews

* remove optimizer, optimizer->pass_manager, make pass a the base class of all

* remove deleted files

* move resolvedependency to sequential pass, use ir_pass namespace

* add todo

* add disabled passes in sequetialpass

* fix minor

* fix currying doc

* remove pass_kind from passnode

* remove pass kind from test

* fix doc

* fix per @tqchen's comments

* remove pass_manager.py create separate classes

* simplify pass_func

* inline using passfunc

* update doc

* disable test_quantize_pass for now

* create PassInfo class to contain the meta data

* flatten passinfo for interface

* retrigger ci

* remove required method

* make Pass python class lighter

* create pass -> decorator

* make the api consistent for all classes

* [relay][frontend] TensorFlow saved model support

* Add Examples section

* keep one copy of tensorflow_parser in relay

* tile and repeat operator added in rely

* fix pylint

* fix make warnings

* comments addressed

* fix lint error

* comment addressed