[functorch] Fix proxy unwrapping for cond(). (#91907)
In control_flow.cond(), we unwrap arguments' proxy by using
get_proxy_slot() call which call a lambda in the end to get the stored
proxy. For SymInt and SymFloat we hide the proxy under a thunk instead
of storing proxy on .proxy attribute diretly, therefore we need to
special case SymInt for unwrapping here.
Fixes #ISSUE_NUMBER
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91907
Approved by: https://github.com/ezyang
