* WIP Run the TF tutorial on TF2

* Remove debugger statement.

* Complete the support for TF2.0's `resize`.

TF2.0 adds a `half_pixel_centers` attribute to the `resize` function in
the image API. This commit completes the hooks in Relay's TF frontend.

At the point of this commit, no new test yet. Also, this commit
addresses solely the `resize` change. Other commits address other
changes in TF2.0.

* Support TF2.0 in the tutorial by using the compat API.

This looks cleaner than trying to detect the TF version.

* Use the TF compat API, so as to support TF2.0.

This is a direct change, relying on the compat API provided by the TF
team.

This code will last as long as the compat API exists, so a
"proper" support for TF1.x and 2.x will require more work in some
future.

* Partial support for EXPLICIT padding introduced in TF2.0.

Explicit padding is a special case in TF2.0 (see reference linked
below). Some models are serialized with that mode, and break TF support
in TVM.

Support is *partial* as EXPLICIT falls back to set padding on the
Relay op, which only supports 2 values. At some point, padding may need
to be extended to support 4 values, but that is out of scope of this
support commit.

Reference on EXPLICIT padding: https://github.com/tensorflow/tensorflow/commit/ec81825aaf7e848d9f8ddffdf1e0d20aebe9172c#diff-1d1c0bb0a880f85b6164f71dbb2f446e

* Guard on checking for optional TF2.0 attribute.

* Do not expect Relay to implement TF-specific attributes.

The `half_pixel_centers` attribute is a new feature in TF2.0. Earlier
commits of mine mistakenly introduce them in the Relay API. This is
probably not what Relay is expected to support, and the semantics of
`half_pixel_centers` is unclear (to me, at least) at this point.

* Remove unclear comment.

CR https://github.com/dmlc/tvm/pull/4104#discussion_r338705742

Addresses #4104

* Changes after review.

Complying without understanding the rationale for now.

* Fix the arguments set mistakenly.

An argument ignored for the wrong operation.