Replace Generator* with Generator that holds std::shared_ptr<GeneratorImpl> (#34468)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34468
This PR prepares `at::Generator` for pybind11's `type_caster<at::Generator>` which is required to implement custom RNG in python. The following changes are done:
1. `at::Generator` was moved to `c10::GeneratorImpl` (similar to `c10::TensorImpl`)
2. `at::Generator` was recreated as a holder of `std::shared_ptr<c10::GeneratorImpl>` (similar to `at::Tensor` that holds `c10::intrusive_ptr<c10::TensorImpl>`)
3. Most of `at::Generator*` usages were replaced with `at::Generator`
TBD: replacing `Generator generator = nullptr` with `{}` requires JIT changes(adding Generator to IValue?)
Differential Revision: D20549420
Pulled By: pbelevich
fbshipit-source-id: 4c92a40eab8f033b359bb6c93f4cd84b07ee8d4e
