Returns an iterator equivalent to map(fn, iter).
Args:
fn: A callable that will take as many arguments as there are
passed iterables.
timeout: The maximum number of seconds to wait. If None, then
there is no limit on the wait time.
chunksize: The size of the chunks the iterable will be broken
into before being passed to a child process. This argument
is only used by ProcessPoolExecutor; it is ignored by
ThreadPoolExecutor.
buffersize: The number of submitted tasks whose results have not
yet been yielded. If the buffer is full, iteration over the
iterables pauses until a result is yielded from the buffer.
If None, all input elements are eagerly collected, and
a task is submitted for each.
Returns:
An iterator equivalent to: map(func, *iterables) but the calls
may be evaluated out-of-order.
Raises:
TimeoutError: If the entire result iterator could not be
generated before the given timeout.
Exception: If fn(*args) raises for any values.