Include `iteration_` in SGD optimizer serialization (#26906)
Summary:
This PR fixes https://github.com/pytorch/pytorch/issues/24192 by including the private field `iteration_` in SGD optimizer serialization. Under the hood, `iteration_` is serialized into an `IValue`, then stored in a JIT module as an attribute.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26906
Differential Revision: D17628359
Pulled By: yf225
fbshipit-source-id: beec1367459e973a1c9080dc86f502e4c7bc5ebd
