Now when initializing a ScriptModule during the torch.jit.load()
process, there is already a cpp module backing the thing. That means
that setting training will overwrite whatever the initialized
ScriptModule had.

This PR splits apart the common "set up internal state" part of the
Module __init__ and calls that from ScriptModule.__init__ and
Module.__init__, leaving the "nn.Module-specific" part (setting
`self.training`) for the nn.Module __init__

ghstack-source-id: 9b2ba8a15c43cf230363e4cd10ba4ad3ac4931f7
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23680