apply offset to indices in multiprocessed map
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Fix #597 

I fixed the indices by applying an offset.
I added the case to our tests to make sure it doesn't happen again.

I also added the message proposed by @thomwolf in #597 

```python
>>> d.select(range(10)).map(fn, with_indices=True, batched=True, num_proc=2, load_from_cache_file=False)
Done writing 10 indices in 80 bytes .
Testing the mapped function outputs
[0, 1]
Testing finished, running the mapping function on the dataset
Done writing 5 indices in 41 bytes .
Done writing 5 indices in 41 bytes .
Spawning 2 processes
[0, 1, 2, 3, 4]
[5, 6, 7, 8, 9]
#0: 100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 1/1 [00:00<00:00, 377.90ba/s]
#1: 100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 1/1 [00:00<00:00, 378.92ba/s]
Concatenating 2 shards from multiprocessing

# Dataset(features: {'label': ClassLabel(num_classes=2, names=['neg', 'pos'], names_file=None, id=None), 'text': Value(dtype='string', id=None)}, num_rows: 10)
```
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