[DataPipe] Simplify BatcherIterDataPipe by removing 'unbatch_level' argument and functionality (#68594)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68594
Based on my conversation with ejguan [here](https://github.com/pytorch/pytorch/pull/68197#pullrequestreview-809148827), we both believe that having the `unbatch_level` argument and functionality is making this DataPipe unnecessarily complicated, because users can call `.unbatch` before `.batch` if they would like to do so. That will likely be cleaner as well.
I also checked other libraries (for example, [TensorFlow](https://www.tensorflow.org/api_docs/python/tf/data/Dataset#unbatch)), and I do not see them provide the ability the `unbatch` within the `batch` function either.
This PR simplifies the DataPipe by removing the argument.
cc VitalyFedyunin ejguan NivekT
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D32532594
Pulled By: NivekT
fbshipit-source-id: 7276ce76ba2a3f207c9dfa58803a48e320adefed
