BucketIterator cannot actually batch examples of similar lengths together unless sort_within_batch=True

[shuaihuaiyi]

🐛 Bug

Describe the bug

text/torchtext/data/iterator.py

Line 280 in ec413ff

def pool(data, batch_size, key, batch_size_fn=lambda new, count, sofar: count,

According to the pool function, especially line 291 to 293, it will not sort examples in a chunk unless sort_within_batch=True. When sort_within_batch=False, it will only act like Iterator. I have tested BucketIterator with sort_within_batch=False, seems like the generated batches are indeed of same length with many <pad>

To Reproduce
Steps to reproduce the behavior:
You don't need to

Expected behavior
it should batch examples of similar lengths together