It should be easier to get a backend instance from a Batch or Session

[nonhermitian]

What is the expected feature or enhancement?
It appears there is no way to get a backend instance from a Batch or Session. But they do have service instances. So if I want to get the backend associated with the Batch or Session I must write:

backend = batch.service.backend(batch.backend())

There should be a better way then this.

Acceptance criteria
It is easier for an user to get a backend instance from a Batch or Session.

[nonhermitian]

If I go down to the sampler or estimator level it gets messier:

backend = sampler.mode.service.backend(sampler.mode.backend())

[jyu00]

Given passing a backend name to Session/Batch constructor is deprecated, we should change session.backend() to return a backend instance. How to make this transition backward compatible is going to be tricky though.