[FORKED] Adding EncT5 model for non-autoregressive tasks 
Author: hackyon
Pull request: yes

# What does this PR do?

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Fixes # (issue)


## Before submitting
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Comments:
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.

cc @ArthurZucker 

Might be a duplicate of #26683

Thanks Amy and Arthur. 

As @ArthurZucker pointed out, I drafted this PR to show a potential alternative to #26683 (to help visualize what the proposed alternative would look like). 

I'll keep this PR in draft mode for now, and will close along with #26683 when the time comes. Thanks!