[Opt Overlap] Clean up code in _OptimizerHookState (#71620)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71620
Remove from_functional_optim and make it the default constructor since
that is the only way _OptimizerHookState is now being built. Also, no longer
need to expose create_functional_optim helper function
ghstack-source-id: 147577174
Test Plan: CI
Reviewed By: cbalioglu
Differential Revision: D33700593
fbshipit-source-id: ba089ce3bf66ccf8f71cffdd0f4d4bddc03e8b14
(cherry picked from commit a50b2caf0e19f9793fbf18b371d30e3dd8c5c0cf)
