[dtensor] PART 2: move DTensor abstraction and APIs to core distributed (#88176)
This PR moves the core DTensor abstraction and high level APIs to
torch.distributed._tensor folder, which includes the following:
1. DTensor class
2. high level APIs (distribute_tensor/module)
3. dispatching logic
4. redistribute logic
part of https://github.com/pytorch/pytorch/issues/88838
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88176
Approved by: https://github.com/fduwjj
