run_seq2seq.py doesn't work after enabling sortish sampler
It gives an error saying **AttributeError: 'Dataset' object has no attribute 'make_sortish_sampler'**. Seems like something is wrong with the pipeline or versions. I install both transformers and datasets from the sources. 

[Exact line](https://github.com/huggingface/transformers/blob/master/src/transformers/trainer_seq2seq.py#L46)
```


    self.train_dataset.make_sortish_sampler(
AttributeError: 'Dataset' object has no attribute 'make_sortish_sampler'



```













## Environment info

- `transformers` version: 4.3.0.dev0
- Platform: Linux-5.8.18-050818-generic-x86_64-with-glibc2.29
- Python version: 3.8.5
- PyTorch version (GPU?): 1.7.1+cu110 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>

### Who can help
@LysandreJik  @patrickvonplaten


