[Relay, OpFusion] Prevent returning a tuple from a fused function #3049
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See #3039 for the context. This is my take on fixing tuple fusion.
a tuple that is an intermediate value in its group can be fused. The existing test case passes unmodified.
a tuple that is the root in its group is lifted out of a subfunction. Before this PR, the tuple is the return value of the subfunction
Here is the approach:
Please review @tqchen @zhiics @vinx13 @MarisaKirisame