[WIP][Unity][TVMScript] return from IfNode

[yongwww]

I am trying to add return support from true/false branch of IfNode as we discussed in tlc-pack/relax#446.

I introduced a RelaxReturnGlobalInfo (similar to DummyGlobalInfo), which is used to save all the Expr (body of SeqExpr ) that need to be returned. But this causes it only works at IR_Module level, it failed to work for relax function without explicitly defining in ir_module. Options I have at this moment.

• o0: Leverage metadata to save all the return Exprs.
• o1: Introduce GlobalInfo in relax Function level, a new member like global_info will be added into FunctionNode
• o2: Add a new member tag return_body into SeqExpr (we don't want to change IR at this stage, list it here for consideration in the future)

Any comments are truly appreciated! cc: @slyubomirsky @tqchen @junrushao @Hzfengsy @YuchenJin

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update: If a pass modifies the return Expr (for example the original return expr is a CallNode, a var is created and bound to the CallNode in a pass, thus the var is the newly generated return expr), then the globally maintained return exprs should be updated in the pass as well. Looks it burdens the pass writing.

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[yongwww]

The happy path tests are working well in my code actually. I close this PR since it should be fine to ask user to return in the end of the function at this moment, I can reopen and keep working on this if needed.