Move generator state APIs to ATen (#49589)
Summary:
## Rationale
While most of the `torch.Generator` properties and methods are implemented as a thin wrapper of the corresponding `at::Generator` methods, `torch.Generator.get_state()` and `torch.Generator.set_state()` are implemented in legacy Torch code and are not dispatched through the `c10::GeneratorImpl` interface. This is not structured well and makes implementing generators for new backends (e.g. `XLAGeneratorImpl` for the XLA backend) inconvenient. As such, this pull request seeks to move these generator state APIs to c10 and ATen.
## What is being refactored?
* Interfaces
- Added `c10::GeneratorImpl::set_state` and `c10::GeneratorImpl::state` for getting and setting the internal state of a random number generator.
- `at::Generator::set_state` and `at::Generator::state` wraps the above-mentioned APIs, as it's basically a PIMPL.
- Added helper function `at::detail::check_rng_state` for checking the validity of new RNG state tensor.
* CPU Generator
- Renamed and moved `THTensor_(setRNGState)` and `THTensor_(getRNGState)` to `CPUGeneratorImpl::set_state` and `CPUGenerator::state`.
- Renamed and moved `THGeneratorState` and `THGeneratorStateNew` to `CPUGeneratorStateLegacy` and `CPUGeneratorState`.
* CUDA Generator
- Renamed and moved `THCRandom_setRNGState` and `THCRandom_getRNGState` to `CUDAGeneratorImpl::set_state` and `CUDAGeneratorImpl::state`.
* PyTorch Bindings
- `THPGenerator_setState` and `THPGenerator_getState` now simply forward to `at::Generator::set_state` and `at::Generator::state`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49589
Reviewed By: H-Huang
Differential Revision: D25785774
Pulled By: pbelevich
fbshipit-source-id: 8ed79209c4ffb1a0ae8b19952ac8871ac9e0255f
