* [Relay] Added MergeComposite pass

This pass allows for patterns to be wrapped
in a function marked with 'Composite' and a
composite function name. This is intended to be
used with the external codegen for the cases where
an external operator maps to multiple Relay
operators. In that case, the mapping can be expressed
as a pattern and assigned a name.

For more information on this pass and its motivation,
see the RFC:
https://discuss.tvm.ai/t/rfc-external-codegen-defining-composite-relay-operators/5470

Change-Id: Icb1b803a9f0ac57c529143200228f3bb5793afc0

* [Relay] Merge composite tests

Added tests for the merge_composite pass.

Change-Id: I1728b4a05b0c1c36140a40f1afe028fde62185dd

* Merge composite additional test

Change-Id: I9bc7d6053c575e9468ac5abc31214c6ad8507e46

* Support priority order in merge_composite

The order in which the patterns are matched
was currently random as an unordered_map was
used to store the pattern table. This uses
arrays instead so that a distinct priority
order of matching can be defined. Additional
tests have also been added to verify this
behaviour.

Change-Id: Ief347df4262639138d5d9d7c8cee7ef233af7b56

* Improved merge composite docs

Change-Id: Ie3a72045ecc3f13ad3c302fbdf192b7296a306a8

* Removed unused variable

Change-Id: I7814d5fde368ffaf1b3d6d806060c774c7720364

* Remove unnecessary op check

Change-Id: I38e78d2acd5b86cb8e837be72ff9d72cd10bcf33

* Improve styling on composite function creation

Change-Id: I37add1c3134e0b5d5085fe1eb9daf8e06890fa8c

* Comment reword

Change-Id: Ie05872dcbbe0c3e1190b0597083b9a64e6b66c66

* Stylistic changes to avoid std::move

Change-Id: I43a93995bbf10530399900c992aa99dd4ae4575f

* Relax a check in ExtractPattern

Change-Id: I0faef77a66c55f83f09e6e47c561ffaea63dedfa

* Remove new line

Change-Id: Ifdd02c12087a7e1a0a9b54825669bc0de8f13c3d

* Removed MatchPattern from MergeComposite

This is not necessary now that ExtractPattern
can fulfill the same purpose.

Change-Id: I14dc020afa8e50f2df4c0a2efb88a011987f8196

* Removed a new line

Change-Id: I8b50f0c9069aa1bcaccbe68eb421031f01a64842

* Improved docs for merge composite

Change-Id: Ib1959a35c856e7ea5639de2e4ef314a54f44caf5

* Fixed free vars in test

Change-Id: I2b7f273db275964ec0e9820560663f0808adee79

* Handle case where root arg might not be a call

Change-Id: I4eeea3ce723d3ba337d110dcc690377daebe8626

* Removed blank line

Change-Id: I07f5392c0e95cfe3cfa5c333703cc6f82d6034fb

* Change to CHECK_EQ

Change-Id: I5c5d62d3cd57f72508b30b926f72091ae6f0d1cc

* Revised a conditional

Change-Id: I23a7897ca15a7cd076db5039dc653a4b8c27e803

* Improved doc styling

Change-Id: I377f0a1c1ac70f3b8d7584b0c49bddc8c6c134ef

* Fail extraction if vars conflict

Change-Id: I78e36d805e8ed6b55e61d490212a967c857554a4

* Added further merge composite tests

Change-Id: Ib1d800409fca4c1834c7fe0cab5a26ab99a26820

Co-authored-by: lhutton1 <35535092+lhutton1@users.noreply.github.com>