[Relay][RFC] FuseOps and Tuple

[MarisaKirisame]

After some consideration I think #2914 is not the best way to support tuple in fusion.
0: it could handle tuple of tuple, it is the relay interpreter that is incorrect (it didnot recursively fset, but instead assume it is either tensor or tuple of tensor).
1: it should not care about reference at all!
as operator accept recursive tuple of tensor, if there is reference in the code, it will never become a tensor (except by function call, or refget). the correct place to catch for reference, is at refget/forigen function call, and not a tensor.

[MarisaKirisame]

@masahi @tqchen

[tqchen]

#3039