[Seq2Seq Templates] Add embedding scale to templates
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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The config.embed_scale parameter is too heavily used in Bart-like models to delete it in future leaner bart versions.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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