Print defaults when using --help for scripts
# What does this PR do?

This PR uses the solution suggested in #12924 to automatically print the defaults of each argument when using `--help` for the script for instance, using `--help` on any of the examples would yield:
```
  --push_to_hub [PUSH_TO_HUB]
                        Whether or not to upload the trained model to the
                        model hub after training.
```
before, and after this PR it will yield
```
  --push_to_hub [PUSH_TO_HUB]
                        Whether or not to upload the trained model to the
                        model hub after training. (default: False)
```

Fixes #12924 
This is indeed a great addition!