[modelcard] Set model name if empty
Author: sanchit-gandhi
Pull request: yes

# What does this PR do?

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When building the model card, if the `model_name` is unspecified we set it to `training_args.output_dir`:
https://github.com/huggingface/transformers/blob/86e435bbb1e54f169351dbb798141afee7fa1b93/src/transformers/modelcard.py#L592-L593

This is typically the case during intermediate push to Hubs during training (when we don't specify any extra push to hub kwargs).

However, if we're fine-tuning from **within** a model repo, we set `--output_dir=./`. This means that `Path(trainer.args.output_dir).name=""`, and so `model_name=""`.

This causes a problem when we try and push the model card to the Hub: a model name of `""` registers as an **empty** model index name, meaning the push is rejected:

```bash
remote: ----------------------------------------------------------        
remote: Sorry, your push was rejected during YAML metadata verification:        
remote: - Error: "model-index[0].name" is not allowed to be empty        
remote: ----------------------------------------------------------        
remote: Please find the documentation at:        
remote: https://huggingface.co/docs/hub/model-cards#model-card-metadata        
remote: ----------------------------------------------------------      
```

This PR sets the `model_name` to `finetuned_from` in the case that it is empty (`""`), meaning that the push to hub is allowed.

Unless there's a neater way of inferring the model repo id, this is probably the best way of preventing rejected push to Hubs?

## Before submitting
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Comments:
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