make _process_dynamic_shapes an implementation detail (#121713)
Summary: `_process_dynamic_shapes` converts new dynamic shapes to old constraints, but in the future may not need to do so. Preparing for that future.
Test Plan: CI
Differential Revision: D54780374
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121713
Approved by: https://github.com/tugsbayasgalan
