Multi-task dataset mixing 
 It seems like many of the best performing models on the GLUE benchmark make some use of multitask learning (simultaneous training on multiple tasks).

The [T5 paper](https://arxiv.org/pdf/1910.10683.pdf) highlights multiple ways of mixing the tasks together during finetuning:
- **Examples-proportional mixing** - sample from tasks proportionally to their dataset size
- **Equal mixing** - sample uniformly from each task
- **Temperature-scaled mixing** - The generalized approach used by multilingual BERT which uses a temperature T, where the mixing rate of each task is raised to the power 1/T and renormalized. When T=1 this is equivalent to equal mixing, and becomes closer to equal mixing with increasing T.

Following this discussion https://github.com/huggingface/transformers/issues/4340 in [transformers](https://github.com/huggingface/transformers), @enzoampil suggested that the `nlp` library might be a better place for this functionality.

Some method for combining datasets could be implemented ,e.g.
```
dataset = nlp.load_multitask(['squad','imdb','cnn_dm'], temperature=2.0, ...)
```

We would need a few additions:
- Method of identifying the tasks - how can we support adding a string to each task as an identifier: e.g. 'summarisation: '?
- Method of combining the metrics - a standard approach is to use the specific metric for each task and add them together for a combined score.

It would be great to support common use cases such as pretraining on the GLUE benchmark before fine-tuning on each GLUE task in turn. 

I'm willing to write bits/most of this I just need some guidance on the interface and other library details so I can integrate it properly.

 
 I like this feature! I think the first question we should decide on is how to convert all datasets into the same format. In T5, the authors decided to format every dataset into a text-to-text format. If the dataset had "multiple" inputs like MNLI, the inputs were concatenated. So in MNLI the input:

> - **Hypothesis**: The St. Louis Cardinals have always won.
> 
> - **Premise**: yeah well losing is i mean i’m i’m originally from Saint Louis and Saint Louis Cardinals when they were there were uh a mostly a losing team but 

was flattened to a single input:

> mnli hypothesis: The St. Louis Cardinals have always won. premise:
> yeah well losing is i mean i’m i’m originally from Saint Louis and Saint Louis Cardinals
> when they were there were uh a mostly a losing team but.

This flattening is actually a very simple operation in `nlp` already. You would just need to do the following:

```python 
def flatten_inputs(example):
    return {"input": "mnli hypothesis: " + example['hypothesis'] + " premise: " + example['premise']}

t5_ready_mnli_ds = mnli_ds.map(flatten_inputs, remove_columns=[<all columns except output>])
```

So I guess converting the datasets into the same format can be left to the user for now. 
Then the question is how we can merge the datasets. I would probably be in favor of a simple 

```python 
dataset.add()
```

function that checks if the dataset is of the same format and if yes merges the two datasets. Finally, how should the sampling be implemented? **Examples-proportional mixing** corresponds to just merging the datasets and shuffling. For the other two sampling approaches we would need some higher-level features, maybe even a `dataset.sample()` function for merged datasets. 

What are your thoughts on this @thomwolf @lhoestq  @ghomasHudson @enzoampil ?