Forces to finish the processing of the current bundle of elements. It will flush
the data cached in the data buffer for processing and retrieves the state mutations (if exists)
made by the Python function. The call blocks until all of the outputs produced by this
bundle have been received.
Executes the Python function with the input element. It's not required to execute
the Python function immediately. The runner may choose to buffer the input element and
execute them in batch for better performance.