decoder_start_token_id was not defined in the config. So generate used bos when it’s not defined in the config. And the batches were simply prepared like this
src_text = "Python was created by <hl> Guido van Rossum <hl> in 1991."
target _text = "Who created python ?"
input_ids = tokenizer(src_text, return_tensors="pt")["input_ids"]
target_ids =  tokenizer(target , return_tensors="pt")["input_ids"]
decoder_input_ids = target_ids[:, :-1].contiguous()
labels = target_ids[:, 1:].clone()