Create an executor backed by a worker pool.

executors A list of tff.framework.Executor instances that forward work to workers in the worker pool. These can be any type of executors, but in most scenarios, they will be instances of tff.framework.RemoteExecutor.
max_fanout The maximum fanout at any point in the aggregation hierarchy. If num_clients > max_fanout, the constructed executor stack will consist of multiple levels of aggregators. The height of the stack will be on the order of log(num_clients) / log(max_fanout).

An instance of executor_factory.ExecutorFactory encapsulating the executor construction logic specified above.