* [Relay][Frontend] Support tf.where

* fix comments

There are many OpenCL platforms that do not yet support OpenCL 2.0,
hence we use 1.2 APIs, some of which are now deprecated.  In order
to turn off the deprecation warnings (elevated to errors by
-Werror) we explicitly disable the 1.2 deprecation warnings.

At the point TVM supports minimum version 2.0, this commit can be
reverted.

* [Relay][Frontend] Support TF Gather

* fix comments

* Fix bias add default axis

* update

* Fix canonicalize ops for bias_add

* error fixed

* rename

* solve conlicts with master

* more test added

* fix error

* remove test

* comment addressed

* gather_nd added

* gather_nd test added

* more test added

* fix lint

* fix build error

* fix lint

* comments addressed

* Update take

* Add special case for canonical simplify and fix test cases

* Use lower case for wrap and clip

* remove unnecssary lower

* Fix mxnet converter for take

* fix

* [DOCKER][FRONTEND] Run DarkNet tests

* update tests to pass CI

* [FRONTEND][TENSORFLOW] bug fix for tensorflow official slim models.

* 	* review comments

* [FRONTEND][ONNX] Some bug fixes and Shape operator fixed for relay.

* 	* test cases

* 	* ci error

Fix comment bugs and code style

This reverts commit f5ca9915.

* do second order

* add comment

* better name

* use tvm assert all close

* refire ci

* [Relay, TOPI] Add deformable conv2d

* Moved to op level2

* Fix lint

* Moved to level2 & bug fix

* Update comments

* Disabled flaky test of conv2d

* Allow converting keras.layers.Sequential

* Use existing new_var function

* Only update expr when missing

* Add test