Add support for custom backend (#95072)
Fixes https://github.com/pytorch/pytorch/issues/92344
A custom backend can be specified by passing in a string with format `"<device_type1>:<backend_name>,<device_type2>:<backend_name>"`, e.g. `"cpu:gloo,cuda:custom_backend"`.
Differential Revision: [D43630050](https://our.internmc.facebook.com/intern/diff/D43630050)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95072
Approved by: https://github.com/kwen2501
