Print default values and introduce ir view classes (#14176)

Print default values and introduce ir view classes (#14176)

Summary:
[Stacked commit, only review the last commit]

This PR adds support for printing default values in python printing as well as the logic
for parsing default values back in using the parser. For simplicity, this PR simply
creates a subgraph of the constant expressions and then runs that graph to generate the defaults.
A more lightweight approach should be possible later, but would require more machinery.

To make reading code in the printer easier, this also add ir_views.h.
Similar to tree_views.h these classes can provide views of some commonly used IR nodes
that have complicated structure and common operations on that structure.

Currently it has only read-only views for prim::If and prim::Loop,
but we should eventually add helpers to manipulate If/Loop nodes as well.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14176

Differential Revision: D13198455

Pulled By: zdevito

fbshipit-source-id: dc99ab9692804ccaedb60a55040c0b89ac7a6a6d