avoid doing quadratic work in concrete type inference (#31020)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31020
Before, the recursive scripting process re-did the concrete type
inference process for every submodule call. This changes things so that
the concrete type inference process only occurs once (at the top level),
and we re-use all the inferred concrete types while recursively
compiling submodules.
This is both more efficient (we don't do n^2 work inferring concrete
types) and less bug-prone (since we infer the concrete type only once,
there is no possibility of a mismatch).
Test Plan: Imported from OSS
Differential Revision: D18904110
Pulled By: suo
fbshipit-source-id: 6560b85ae29fe5e9db1ee982dbf8bc222614b8d8
