maximum_decoding_length
in the parameters: https://opennmt.net/OpenNMT-tf/configuration.html. Maybe length_penalty
can also help.
replace_unknown_target
uses the model attention to select the corresponding source token. However, it is well known that Transformer attention usually can not be used as target-source alignments. You should either constrain the attention to be an alignment or use subword tokenization (like SentencePiece) to avoid UNK. Note that the UNK token does not appear in the vocab but is automatically added when starting the training.
max_step
in the training parameters. There should be a warning about this somewhere in the logs. We just improved that for the next version: a more visible error message will be shown, see OpenNMT/OpenNMT-tf@21df1c7
case_markup
option from the Tokenizer.