Add run_batch to Sampler interface

[kevinsung]

This would be similar to run_async, in that it simply defers to run or run_sweep, and should be overridden by parent classes that can sample batches more efficiently. It would be overridden in QuantumEngineSampler.

If this should be done, I'm willing to work on it.

[zaqqwerty]

Great to see this issue opened, wanted to say that for samplers intended to be used with TFQ, run_batch would be a really helpful addition to the API.

[balopat]

Discussed on Cirq Cynq - this sounds like a good idea. Please go ahead!