Default value of `num_epochs` in `IndexSampler` is unclear

It is unclear from the docs and API reference what `num_epochs=None` (default) actually means for `IndexSampler`.
I suggest to specify it at least in the [API page of `IndexSampler`](https://google-grain.readthedocs.io/en/latest/_autosummary/grain.samplers.IndexSampler.html#grain.samplers.IndexSampler) and perhaps in the `DataLoader` guide.