Clearer indication for overridden method in generation
The expectation for the `prepare_inputs_for_generation` function to be overridden can be made clearer by changing https://github.com/huggingface/transformers/blob/700cee344691afc41f68aa18fedea463b22f95f1/src/transformers/generation_utils.py#L369-L374

to raise a `NotImplementedError` that provides the information mentioned in the function's comment.

@patrickvonplaten 
Also putting this in the "Fix generation docs" task basket<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.