[torch.library] Add ability to create library fragments (#98439)
In C++ we have TORCH_LIBRARY_FRAGMENT. This PR adds the same
functionality to the Python torch.library API.
The motivation for this is: for the simple custom op API, we don't want
users to need to deal with Library objects. One way to hide this from
users is to create library fragments.
Test Plan:
- tests that you can create multiple fragments and def+impl operators on each.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98439
Approved by: https://github.com/ezyang, https://github.com/bdhirsh
