concatenate_datasets support axis=0 or 1 ？
@lhoestq, we have two Pull Requests to implement:
- Dataset.add_item: #1870
- Dataset.add_column: #2145
which add a single row or column, repectively.

The request here is to implement the concatenation of *multiple* rows/columns. Am I right?

We should agree on the API:
- `concatenate_datasets` with `axis`?
- other Dataset method name?
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)
 
 @lhoestq, we have two Pull Requests to implement:
- Dataset.add_item: #1870
- Dataset.add_column: #2145
which add a single row or column, repectively.

The request here is to implement the concatenation of *multiple* rows/columns. Am I right?

We should agree on the API:
- `concatenate_datasets` with `axis`?
- other Dataset method name?