* [NODE][IR] Introduce StructuralEqual Infra for the Unified IR.

This PR introduces a new way to handle structural equality
for both TIR and relay nodes in an extensive way.

- Each object can now register an optional SEqualReduce function, which
  describes how to reduce its structural equality to another instance
  into equality of the children.
- Optionally, the object can choose to allow remapping of vars(e.g. function parameters)
  by calling DefEqual
- We implemented a non-recursive structural equality checker that
  recursively traverses the objects and does the structural equality checking.

This PR also fixes a few potential problems in previous relay's AlphaEqual.

- In particular, the new structural equality relation will be communicative.
- It is can be dangerous to use same_as relation to quickly check equality,
  demonstrated by the following case. (%x, %y) are shared vars between two functions.

- function0: fn (%x, %y) { %x + %y }
- function1: fn (%y, %x) { %x + %y }

The new structural equal is intented to supersede AlphaEqual and AttrsEqual.

Follow-up PRs should be performed to redirect the existing usages, and removes
the corresponding implementation.

* Update the rule to distinguish between graph node and non-graph nodes.

* Refactor the test cases to use structural equal.

* address comments

* Mark more relay::Expr as graph node, fix a testcase issue(was bug that was not caught by previous alpha equal)

* Remove unrelated comment

* Fix file comment

* Address review comment

* Relax condition to fit flaky case