text: Title: add Flash Attention Support for Helsinki-NLP/opus models
  Body: ### Feature request
  
  I would like to propose adding support for flash attention to the Helsinki-NLP/opus models in the Hugging Face library. Judging by what I am seeing [here](https://github.com/huggingface/transformers/blob/main/src/transformers/models/marian/modeling_marian.py), it seems not implemented.
  
  ### Motivation
  
  **performance gains:** adding flash attention would allow for significant speedups
  
  ### Your contribution
  
  I am happy to help test the implementation if needed. Thank you for considering this enhancement, and I look forward to the discussion!