Auto modelcard
# What does this PR do?

This PR adds functionality in the Trainer to auto-generate model cards and some utilities to do the same without the Trainer if people are not using it. In passing, the old `ModelCard` class is deprecated (to be removed in v5).

As an example [here](https://huggingface.co/sgugger/test-glue-mrpc) is a repo that is generated by the `run_glue` script with this new functionality, using the following command on a machine with 2 GPUs:

```
accelerate launch examples/pytorch/text-classification/run_glue.py \
    --model_name_or_path bert-base-cased \
    --task_name mrpc \
    --do_train \
    --do_eval \
    --learning_rate 2e-5 \
    --per_device_train_batch_size 16 \
    --per_device_eval_batch_size 16 \
    --evaluation_strategy epoch \
    --logging_strategy epoch \
    --weight_decay 1e-2 \
    --output_dir ~/tmp/test-glue-mrpc \
    --overwrite_output_dir \
    --push_to_hub
```

I've only adjusted the glue example for now, will do the others once we have settled on an API.