Documentation of nnUNet in monai.apps has several mistakes

**Describe the bug**

- Examples are not correct in https://github.com/Project-MONAI/MONAI/blob/dev/monai/apps/nnunet/__main__.py
- Some commands are using `nnUNetRunner` which is not implemented
- `num_gpus` is not effective

