[c10d] Add is_backend_available for c10d backend. (#101945)
Add is_backend_available for c10d backend, either the built-in backends or third-party backends through function ``Backend.register_backend``.
There is a related discussion in https://github.com/pytorch/pytorch/pull/101775#discussion_r1199253553
> For example in python constructor for their backend they should explicitly add the is_X_available. Or if defining in C++ they should modify pybind like this https://github.com/H-Huang/torch_collective_extension/blob/main/custom_backend/include/dummy.hpp#L98-L101
to also add their own is_available property
It is a natural choice for users to add their own `is_available` when they create a backend. We think it might be a possible way for the user to use `is_X_available` in the same way as the native, for example by dynamically adding`torch.distributed.is_dummpy_available()` function. This is why we want to dynamically add the `is_X_available` to `torch.distributed` in `register_backend`.
> Or we could add an Is_available(backend) function, that checks for the backend.
Providing a public function is indeed another good approach. We have implemented an `is_backend_available` in https://github.com/pytorch/pytorch/pull/101945 that supports both built-in backends and third-party backends.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101945
Approved by: https://github.com/H-Huang
