replaced call to _prepare_decoder_attention_mask() with _prepare_4d_causal_attention_mask()

[poedator]

fixes issue #536
edited calls to mask methods consistently with huggingface/transformers#27086

[justheuristic]

Hi!

Note: a day ago, we had suspicions that the issue is with the main branch and not this one. It appears not to be the case.

I can confirm that def test_input_ids, specifically, passes in the main branch but fails here. This is most likely a bug. I can look into this eventually, but if you have time, please look into this yourself first.