Sets context which compiles to and executes mergeable comp form.

async_contexts Asynchronous TFF execution contexts across which to distribute work.
num_subrounds An optional integer, specifying total the number of subrounds desired. If unspecified, the length of async_contexts will determine the number of subrounds. If more subrounds are requested than contexts are passed, invocations will be sequentialized. If fewer, the work will be run in parallel across a subset of the async_contexts.