[examples/flax] use Repository API for push_to_hub
# What does this PR do?

This PR updates all flax examples scripts to use the `Repository` API for `push_to_hub` and removes all instructions to manually create/clone a repo in the READMEs.
> Thanks a lot for taking care of this - can we leave the symbolic links though so that people can continue following the steps like a recipe?

@patrickvonplaten the issue with this is that, we need to pass "./" or "." as the `output_dir` which does not work with `Repository`. 

What these changes do is that they create a directory for model/tokenize/config in `/tmp` and uses that path to save the `tokenizer` and `config` and use that as the `output_dir`.  So the users should still be able to follow the steps only difference is the script will nbe run from its own directory rather than creating a symlink.<|||||>Thanks, @LysandreJik for pointing that out, we could change the path from `/tmp` to something else. Some of our PyTorch examples also use `/tmp`, so we should also correct those no?

> Also Repository(".", clone_from="xxx") should work!

It does! Issue is 

```python
 if training_args.hub_model_id is None:
            repo_name = get_full_repo_name(Path(training_args.output_dir).name, token=training_args.hub_token)
        else:
            repo_name = training_args.hub_model_id
        repo = Repository(training_args.output_dir, clone_from=repo_name)
```

if `hub_model_id` is not passed, `repo_name` is derived from `output_dir`, and when the path is `"."`, then `path.name` returns an empty string, which then results in this error
`requests.exceptions.HTTPError: 400 Client Error: Bad Request for url: https://huggingface.co/api/repos/create - Only regular characters and '-', '_', '.' accepted`<|||||>Maybe the `Path(training_args.output_dir).name` should be updated to `Path(training_args.output_dir).absolute().name` in that case?<|||||>Aah, yeah! Thanks Sylvain :) 