Fix: Make `__len__` of datapipes dynamic (#88302)
Fixes #88074
Several datapipes have their lengths cached on being executed for the first time. However, source datapipes might change in length (most prominently, whenever `apply_sharding` is called). The behaviour is counter-intuitive because we do not expect `__len__` to have side-effects.
This PR makes `__len__` dynamically computed.
Changes:
- Add note to the `datapipes` README that `__len__` should be dynamic and why.
- Remove caching of length computations in `ConcaterIterDataPipe`, `MultiplexerIterDataPipe`, `ZipperIterDataPipe`, `BatcherIterDataPipe`, `ConcaterMapDataPipe`, and `BatcherMapDataPipe`.
- This required removal of the `length` attribute in setstate/getstate of `MultiplexerIterDataPipe`. I am unsure whether to remove this completely and risk breaking saved checkpoints (as I did) or whether to just ignore the `length` of the loaded `state`.
- This also means the classes above no longer have a `length` attribute. I have found no uses of this, though.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88302
Approved by: https://github.com/NivekT
