Populate the backend name of cpu DELEGATE_CALLs (#1749)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1749

DELEGATE_CALL is a special CPU event corresponding to a delegate call.

Technically it is not a delegated op, so the backend name is not populated, but it is beneficial for the name to be populated.
This diff adds an exception for this event's backend name to be populated.

Reviewed By: tarun292, navsud

Differential Revision: D53221517

fbshipit-source-id: 2e0898c0b3af51c63d7ceccf6540b0902ef6461e