concatenate_datasets fails when two dataset with shards > 1 and unequal shard numbers
Hi ! Thanks for reporting :)

I managed to reproduce the hub using
```python

from datasets import concatenate_datasets, Dataset, load_from_disk

Dataset.from_dict({"a": range(9)}).save_to_disk("tmp/ds1")
ds1 = load_from_disk("tmp/ds1")
ds1 = concatenate_datasets([ds1, ds1])

Dataset.from_dict({"b": range(6)}).save_to_disk("tmp/ds2")
ds2 = load_from_disk("tmp/ds2")
ds2 = concatenate_datasets([ds2, ds2, ds2])

concatenate_datasets([ds1, ds2], axis=1)
```
and I get
```python
Traceback (most recent call last):                                                                                                    
  File "test.py", line 98, in <module>
    dds = concatenate_datasets([ds1, ds2], axis=1)
  File "/Users/.../datasets/combine.py", line 182, in concatenate_datasets
    return _concatenate_map_style_datasets(dsets, info=info, split=split, axis=axis)
  File "/Users/.../datasets/arrow_dataset.py", line 5499, in _concatenate_map_style_datasets
    table = concat_tables([dset._data for dset in dsets], axis=axis)
  File "/Users/.../datasets/table.py", line 1778, in concat_tables
    return ConcatenationTable.from_tables(tables, axis=axis)
  File "/Users/.../datasets/table.py", line 1483, in from_tables
    blocks = _extend_blocks(blocks, table_blocks, axis=axis)
  File "/Users/.../datasets/table.py", line 1477, in _extend_blocks
    result[i].extend(row_blocks)
IndexError: list index out of range
```

It appears to happen when the two datasets have a number of shards that is not the same
### Describe the bug

When using `concatenate_datasets([dataset1, dataset2], axis = 1)` to concatenate two datasets with shards > 1, it fails:

```
File "/home/xzg/anaconda3/envs/tri-transfer/lib/python3.9/site-packages/datasets/combine.py", line 182, in concatenate_datasets
    return _concatenate_map_style_datasets(dsets, info=info, split=split, axis=axis)
File "/home/xzg/anaconda3/envs/tri-transfer/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 5499, in _concatenate_map_style_datasets
    table = concat_tables([dset._data for dset in dsets], axis=axis)
File "/home/xzg/anaconda3/envs/tri-transfer/lib/python3.9/site-packages/datasets/table.py", line 1778, in concat_tables
    return ConcatenationTable.from_tables(tables, axis=axis)
File "/home/xzg/anaconda3/envs/tri-transfer/lib/python3.9/site-packages/datasets/table.py", line 1483, in from_tables
    blocks = _extend_blocks(blocks, table_blocks, axis=axis)
File "/home/xzg/anaconda3/envs/tri-transfer/lib/python3.9/site-packages/datasets/table.py", line 1477, in _extend_blocks
    result[i].extend(row_blocks)
IndexError: list index out of range
```

### Steps to reproduce the bug

dataset = concatenate_datasets([dataset1, dataset2], axis = 1)

### Expected behavior

The datasets are correctly concatenated.

### Environment info

datasets==2.8.0