Grain 0.2.15

• New features:
  • Adds public DatasetIterator.close API as an explicit blocking alternative
    to a cleanup during GC.
  • DatasetIterator.start_prefetch now propagates to the first asynchronous
    parent iterator instead of raising NotImplemented. This API is useful for
    hiding first batch processing behind model checkpoint recovery.
  • Introduces grain.experimental.multithread_prefetch as an
    alternative to multiprocessing prefetch in free-threading Python.
  • Adds experimental support for static {Map|Iter}Dataset element
    specification inference.
  • Adds support for changing IterDataset.mix components and weights after a checkpoint.