Make lower-level utilities available to third-party frameworks

[speller26]

Current Problem
I want to use some of the the low-level utilities provided in dwave-cloud-client directly from a third-party sampler.

For example, I'd like to validate that my problem is compatible with a device's graph. This functionality is available in the DWaveSampler's underlying Client, but not directly from the sampler; I have to access the low-level Client. However, a third-party sampler does not have an underlying Client, so it becomes impossible to use this feature.

Proposed Solution
Expose useful lower-level utilities through the Sampler interface.

[arcondello]

I think there are two specific features we'd like to support:

1. Graph validation. This can be done now with

all(v in sampler.adjacency for v in bqm.variables) and (u in sampler.adjacency[v] for u, v in bqm.quadratic)

if you have a StructuredSampler.

One solution might be to add a sampler.check_structure(bqm) method to the structured sampler ABC.

2. Variable format massaging. See Expose reformatted parameters dwave-cloud-client#465

[speller26]

When you say StructuredSampler, do you just mean that the Sampler implementation should subclass both Sampler and Structured (I don't see a StructuredSampler class; will that be added)?

[arcondello]

Yes, it should subclass both, like the DWaveSampler.

[randomir]

To @speller26's point, adding StructuredSampler with relevant utilities, and then subclassing it in DWaveSampler makes the most sense to me (see dwavesystems/dimod#832 (comment)).

Something like:

# in dimod
class StructuredSampler(dimod.Structured, dimod.Sampler):
    def samples_from(self, bqm):
        ...

# in dwave-system
class DWaveSampler(dimod.StructuredSampler):
    ...