Include decoder_attention_mask in T5 model inputs
# What does this PR do?

This PR includes decoder_attention_mask as an argument in the prepare_inputs_for_generation function, helping enable the use of custom attention masks in the decoder.

Duplicate of https://github.com/huggingface/transformers/pull/22819

@gante @amyeroberts 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@amyeroberts Could you merge this PR please?