

TITLE
Clearer indication for overridden method in generation

COMMENTS
2

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BODY
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 