Fix types on Sampler.run_async (#1847)
Fix types on Sampler.run_async (#1847)

`async` methods should be annotated with the type you get when `await`-ing the coroutine; mypy knows to wrap this with `Awaitable` when checking callsites. See https://mypy.readthedocs.io/en/stable/more_types.html#async-and-await.
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,balopat/Cirq,balopat/Cirq