[functorch] Fix expand.SymInt issues by adding a decomp (pytorch/functorch#903)
To fully support expand.SymInt, we should write a batching rule for it.
Unfortunately, we are unable to write a batching rule because there are
not enough SymInt overloads (e.g. we need a view.SymInt, that doesn't
exist today).
This PR adds a decomp for expand.SymInt as a short-term fix.
