Allow a StreamingDataset to wrap around when running in a CombinedStreamingDataset

Nobody has claimed this yet.

## 🚀 Feature

Consider adding the ability to _wrap_ around a StreamingDataset without issuing a StopIteration when combining datasets.

This is something we haven't ported from PackedDataset https://github.com/Lightning-AI/lit-llama/blob/main/lit_llama/packed_dataset.py#L190

### Motivation

This is useful to combine a smaller dataset with a very large one, so that we can make sure certain batches of data make it into the training process frequently enough, but without invalidating the epoch when the other datasets are multi-billion tokens in size.

### Pitch

Add a `wrap` property to a StreamingDataset that will not have it raise a StopIteration but just keep looping through the data.

### Alternatives

Add handling of this at the CombinedStreamingDataset level, so that each dataset raises StopIteration when it has to, but we don't invalidate the others. In both cases we need to decide what happens to `epoch` within the dataset.

Start by reading the StreamingDataset and CombinedStreamingDataset implementations, then compare their behavior with the linked PackedDataset wrap logic. Decide whether wrapping belongs on the individual dataset or the combined dataset, including how epoch should behave; done means a smaller dataset can repeat without raising StopIteration while combined iteration remains valid.

Written by the indexing model from the issue text.