Why are these changes needed?
Consumers (e.g. Train) may expect generated batches to be of the same size. Prior to this change, the default behavior would be for each batch to be one block, which may be of different sizes.
Changes
Set default batch_size to 256. This was chosen to be a sensible default for training workloads, which is intentionally different from the existing default batch_size value for Dataset.map_batches.
Update docs for Dataset.iter_batches, Dataset.map_batches, and DatasetPipeline.iter_batches to be consistent.
Updated tests and examples to explicitly pass in batch_size=None as these tests were intentionally testing block iteration, and there are other tests that test explicit batch sizes.
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