Add `CheckpointIO` classes to split checkpoints

Nobody has claimed this yet.

## 🚀 Feature

Create new `CheckpointIO` classes that allow:
- Splitting a checkpoint by keys, enabling #5339
- Splitting a checkpoint by size, relaxing the memory and storage constraints

### Motivation

See existing discussions in https://github.com/huggingface/transformers/issues/13548 for why these are interesting features to end-users.

### Pitch

`Trainer(plugins=CheckpointKeySplitterIO())` or `Trainer(plugins=CheckpointSizeSplitterIO(max_size="10GB"))`

### Alternatives

Have users create and maintain these solutions. The `Trainer` should allow it.

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cc @borda @tchaton @justusschock @awaelchli @jjenniferdai @rohitgr7

Start with the Trainer plugin entry point and review the linked discussion in issue #13548 for the intended use cases. Define the CheckpointIO interfaces for splitting checkpoints by keys and by size, then make the proposed Trainer plugin configurations work and verify that both splitting modes meet their stated goals.

Written by the indexing model from the issue text.