Make datasets in `ConcatDataset` not need to be sized (#64114)
Summary:
`datasets` needs to be iterable, but also sized because the length is checked. But immediately after it's converted to a list. By changing the order of these 2 lines, it doesn't need to be sized anymore.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64114
Reviewed By: H-Huang
Differential Revision: D30641480
Pulled By: ejguan
fbshipit-source-id: 7e16548c2123afa65b83845f9929271fa07fe1e8
