* [RELAY][BYOC] Register pattern tables from external codegens

This adds utility functions to support registering
and retrieving pattern tables used by MergeComposite for
external codegens.

Change-Id: I5be165a321440e48b15ff6aff4970e0c67496aaa

* Updated DNNL tests to use pattern table mechanism

* Removed pattern table standalone test

* Change reg to _op