Documentation: Dataset to Model interface examples
# 🚀 Feature request

Add explicit examples to the custom dataset docs which use fully realized key-value pairs and link to the forward methods of the relevant models to emphasize the contract between Dataset `__getitem__()` output and model `forward()`.


## Motivation

This issue arises when you consider the interaction of Datasets and Models. They may be independently well documented but when trying to use them together there are gaps.

Example:

You want to fine tune T5 using `Trainer` using a custom `Dataset`.

The minimal API for T5 a user needs to be aware of in this scenario is small but not well documented in the context of custom datasets. The `Dataset` must return an object in this format from it's `__getitem__()` method.

```
        return {
            'input_ids': input_ids, 
            'attention_mask': attention_mask, 
            'labels': labels,
        }
```

Those three keys are then passed by `Trainer` to T5 via its forward method. Arguably those parameters are the most important first interface you need to know about to train T5 (inside or outside of `Trainer`). 

This *is* documented in the T5 docs, however the key disconnect is that in the custom dataset docs none of the examples use explicit, fully realized set of key-value pairs. Also the docs don't emphasize that those return values *must match* the expected inputs to a model's `forward()` method.

## Your contribution

I can add these if this get's a few thumbs up.

The custom dataset doc page is outdated and will be rewritten soon, you should use the [examples scripts](https://github.com/huggingface/transformers/tree/master/examples) or [example notebooks](https://huggingface.co/transformers/notebooks.html) as a base to fine-tune models.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.