Confused by description: encoder_no_repeat_ngram_size

I’m confused as to what the encoder_no_repeat_ngram_size argument does. The documentation describes it like so:

Value that will be used by default in the generate method of the model for encoder_no_repeat_ngram_size . If set to int > 0, all ngrams of that size that occur in the encoder_input_ids cannot occur in the decoder_input_ids .

I understand that decoder_input_ids are labels for training the decoder, what’s their purpose in generation?