…re calling set_parallel_chunksize.

…size, 2) set chunksize back to the default of 0 and then after the gufunc returns, restore the chunksize back to the previously saved value. This way, the current thread gets its default chunksize behavior inside the parallel region but goes back to its previous value when the region is over.

…its use point.

Co-authored-by: stuartarchibald <stuartarchibald@users.noreply.github.com>

Co-authored-by: stuartarchibald <stuartarchibald@users.noreply.github.com>

More details on how actual chunksize can differ from specification.
Moved code examples in docs to tests/doc_examples/test_parallel_chunksize.py.
Export (g,s)et_parallel_chunksize from numba.np.ufunc.
Fix withcontext parallel_chunksize doc string.
Change set_parallel_chunksize to return previous chunk size.
Use that return value to remove need for get_parallel_chunksize in some places.
Raise exception if negative value to set_parallel_chunksize.

This:

* Makes `_get_thread_id()` return enumerated threads ids from
  0..mask.
* Makes it possible to obtain the parfors schedule from within a
  parallel region.
* Implements broadcasting of schedule info across all threads
  for convenience.