Add API to read value of `SymInt` and`ParamsBuffer` (#5754)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5754

## Context

This diff adds an API to read the value from a `SymInt`. This functionality will be useful because `SymInt`s may be needed to set tensor sizes, in addition to being used as arguments to shaders.
ghstack-source-id: 245517196
exported-using-ghexport

Reviewed By: jorgep31415

Differential Revision: D63642093

fbshipit-source-id: 14ce9daecd5520cf078d3b3259eaafb5b654d834