Attribute serialization improvements (#18188)

Attribute serialization improvements (#18188)

Summary:
* adds attributes to `ScriptModule.__getattr__` so they can be accessed in Python after re-importing
* full support for all the possible values for an `int64_t`
    * this necessitated a bunch more `pushWhatever` functions, so re-introduced a templated version to cut down on duplicate code
* tests to validate references / value sharing works
* adds `torch.jit.Unpickler` which people can use to de-serialize the pickle files into Python / have a quick reference on how to do this without PyTorch
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18188

Differential Revision: D14527490

Pulled By: driazati

fbshipit-source-id: efd15579cc04aa2e28c4b2c9490d82d849dee559