[Unity] NestedMsg Support utility (#13995)

This PR introduce NestedMsg to robustly handle nested-tuple analysis.

Relax support nested tuple structures in the IR.
Nested tuple structure is important to support advanced groupings in
cases such as gradient calculation and other scenarios.

The possible presence of nested tuple does mean that we need to to
robustly handle analysis that contains nested tuple structures in a dataflow graph.

This PR introduces a NestedMsg<T> class that corresponds to a possibly
nested message tuple for a given leaf message class T.
We also introduces various helper functions to compose and decompose messages.

Co-authored-by: Bohan Hou <32121147+spectrometerHBH@users.noreply.github.com>
Co-authored-by: Yixin Dong <ubospica@gmail.com>
Co-authored-by: Ruihang Lai <ruihangl@cs.cmu.edu>