* Add a default decoder_attention_mask for EncoderDecoderModel during training
Since we are already creating the default decoder_input_ids from the labels, we should also
create a default decoder_attention_mask to go with it.
* Fix test constant that relied on manual_seed()
The test was changed to use a decoder_attention_mask that ignores padding instead (which is
the default one created by BERT when attention_mask is None).
* Create the decoder_attention_mask using decoder_input_ids instead of labels
* Fix formatting in test
a0fd34483f