changes to MappedExamplesIterable to resolve #7345
I noticed that `Dataset.map` has a more complex output depending on `remove_columns`. In particular [this](https://github.com/huggingface/datasets/blob/6457be66e2ef88411281eddc4e7698866a3977f1/src/datasets/arrow_dataset.py#L3371) line removes columns from output if the input is being modified in place (i.e. `input_columns = None`). I tried to mimic this behaviour in `MappedExamplesIterable` by checking if the input and output point to the same dictionary object. The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7353). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.
modified `MappedExamplesIterable` and `test_iterable_dataset.py::test_mapped_examples_iterable_with_indices`

fix #7345

@lhoestq 