`dataset = dataset.map()` causes faiss index lost

## Describe the bug
assigning the resulted dataset to original dataset causes lost of the faiss index

## Steps to reproduce the bug
`my_dataset` is a regular loaded dataset. It's a part of a customed dataset structure
```python
self.dataset.add_faiss_index('embeddings')
self.dataset.list_indexes()
# ['embeddings']

dataset2 = my_dataset.map(
lambda x: self._get_nearest_examples_batch(x['text']), batch=True
)

# the unexpected result:
dataset2.list_indexes()
# []

self.dataset.list_indexes()
# ['embeddings']
```

in case something wrong with my `_get_nearest_examples_batch()`, it's like this
```python
def _get_nearest_examples_batch(self, examples, k=5):
queries = embed(examples)
scores_batch, retrievals_batch = self.dataset.get_nearest_examples_batch(self.faiss_column, queries, k)
return {
'neighbors': [batch['text'] for batch in retrievals_batch],
'scores': scores_batch
}
```

## Expected results
`map` shouldn't drop the indexes, in another word, indexes should be carried to the generated dataset

## Actual results
map drops the indexes

## Environment info

- `datasets` version: 1.18.3
- Platform: Ubuntu 20.04.3 LTS
- Python version: 3.8.12
- PyArrow version: 7.0.0