Remove const_cast's from subgraph matcher. (#20303)
Summary:
The trick here is that creating a mapping from const values to
const values means that downstream clients that want to mutate
the output of the mapping are stuck. However, a mapping from
const values to non-const values is just fine and doesn't put
constraints on downstream clients.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20303
Differential Revision: D15284076
fbshipit-source-id: 16206fd910dd5f83218525ca301b1889df0586cb
