Add prefetching into parallel_interleave

This change adds 2 parameters to parallel_interleave:
 - prefetch_input_elements: determines the number of iterators to prefetch
     allowing buffers to warm up and data to be pre-fetched without blocking
     the main thread (i.e. the GetNext() call).
 - buffer_output_elements: in order to avoid creating thousands of threads, we
     fuse in the .prefetch() operator as an additional parameter. The value of
     this parameter is identical to the value passed to `.prefetch()`

PiperOrigin-RevId: 179726088