Enable len(dataloader) for iterable dataset (#23587)
Summary:
Copy-paste comment from code for reasoning:
```
# NOTE [ IterableDataset and __len__ ]
#
# For `IterableDataset`, `__len__` could be inaccurate when one naively
# does multi-processing data loading, since the samples will be duplicated.
# However, no real use case should be actually using that behavior, so
# it should count as a user error. We should generally trust user
# code to do the proper thing (e.g., configure each replica differently
# in `__iter__`), and give us the correct `__len__` if they choose to
# implement it (this will still throw if the dataset does not implement
# a `__len__`).
#
# To provide a further warning, we track if `__len__` was called on the
# `DataLoader`, save the returned value in `self._len_called`, and warn
# if the iterator ends up yielding more than this number of samples.
```
Fixes https://github.com/pytorch/pytorch/issues/30184
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23587
Differential Revision: D18852625
Pulled By: ailzhang
fbshipit-source-id: aea8d4d70c7f21aaa69b35908a6f43026493d826
