concatenate_datasets support axis=0 or 1 ？
Actually it's doable but requires to update the `Dataset._data_files` schema to support this.
I'm re-opening this since we may want to add this in the future
I want to achieve the following result
![image](https://user-images.githubusercontent.com/12437751/99207426-f0c8db80-27f8-11eb-820a-4d9f7287b742.png)

concatenate_datasets support axis=0 or 1 ？ 
 I want to achieve the following result
![image](https://user-images.githubusercontent.com/12437751/99207426-f0c8db80-27f8-11eb-820a-4d9f7287b742.png)
 
 Actually it's doable but requires to update the `Dataset._data_files` schema to support this.
I'm re-opening this since we may want to add this in the future