CLI dummy data generation is broken
## Describe the bug
We get a TypeError when running CLI dummy data generation:
```shell
datasets-cli dummy_data datasets/<your-dataset-folder> --auto_generate
```
gives:
```
  File ".../huggingface/datasets/src/datasets/commands/dummy_data.py", line 361, in _autogenerate_dummy_data
    dataset_builder._prepare_split(split_generator)
TypeError: _prepare_split() missing 1 required positional argument: 'check_duplicate_keys'
```

