Remove unnecessary attr assignment
Author: tugsbayasgalan
Pull request: yes

# What does this PR do?

This PR removes unnecessary attr assignment that make torch.export fail. Since I couldn't find anywhere where this is actually used, i think it is ok to delete it. 
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Fixes # (issue)


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Comments:
cc: @guangy10 

Hi 👋, thank you for opening this pull request! The pull request is converted to draft by default. When it is ready for review, please click the `Ready for review` button (at the bottom of the PR page).

@tugsbayasgalan Can you add test for this model? If the fix will make it exportable. Particularly I'd like to see how we actually export this model. This is another encoder-decoder model for seq2seq generation, similar to T5. I tried export the [pegasus model](https://github.com/huggingface/transformers/blob/9be4728af8bec48073ae841881d7f4e2ac3521d1/src/transformers/models/pegasus/modeling_pegasus.py) using the similar setup I created for T5 (#36486), however I have trouble to get its decoder exported with `Cache`.