Extreme inefficiency for `save_to_disk` when merging datasets

### Describe the bug

Hi, I tried to merge in total 22M sequences of data, where each sequence is of maximum length 2000. I found that merging these datasets and then `save_to_disk` is extremely slow because of flattening the indices. Wondering if you have any suggestions or guidance on this. Thank you very much!

### Steps to reproduce the bug

The source data is too big to demonstrate

### Expected behavior

The source data is too big to demonstrate

### Environment info

python 3.9.0
datasets 2.7.0
pytorch 2.0.0
tokenizers 0.13.1
transformers 4.31.0