...resume
Fixes huggingface#7630
### Problem
When calling `.map()` on an `IterableDataset`, resuming from a checkpoint skips a large number of samples. This is because `MappedExamplesIterable` did not implement `state_dict()` or `load_state_dict()`, so checkpointing was not properly delegated to the underlying iterable.
### What This PR Does
This patch adds:
```python
def state_dict(self):
 return self.ex_iterable.state_dict()
def load_state_dict(self, state):
 self.ex_iterable.load_state_dict(state)
to MappedExamplesIterable, so the wrapped base iterable's state can be saved and restored as expected.
Result
Using .map() no longer causes sample skipping after checkpoint resume.
Tested manually using the reproducer script in huggingface#7630. The resumed iterator now yields the correct samples.
Let me know if a dedicated test case is required — happy to add one!