Why is terratorch_task required in tasks.

I define the `terratorch_task` in `defaults`. It's the same for every task. However, when I don't define `terratorch_task` in `tasks`, I get this error:
```
argparse.ArgumentError: Validation failed: Key "terratorch_task" is required but not included in config object or its value is None.
```

Would be great if it is only required if it is not defined in defaults.