[`Pegasus(-x)`] Fixup embedding interfaces on Encoder/Decoder
Author: vasqu
Pull request: yes

# What does this PR do?
Discovered through a fluke tbh but it seems some modelling code is missing for the pegasus and pegasus-x modelling files. This only relates to getting, setting, and or resizing the token/positional embeddings on the encoder/decoder parts of the whole model. 

It's nothing big and I doubt many even use the pegasus models atp but I thought I'd share this nonetheless.
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@ArthurZucker 

Comments:
Ok looked into other models like t5 and bart who also share their embeddings and they also allow setting/getting embeddings on each part (encoder, decoder). 

It really seems like an oversight rather than intention that it is missing here. It's further supported by the fact that only the decoder allows setting/getting embeds but the encoder does not - if we really were about ensuring the shared state then setting would be disallowed in any case imo.

BTW @vasqu do you have an email I can reach out to? 🤗 

@ArthurZucker Sure, either [antonprogamer@gmail.com](mailto:antonprogamer@gmail.com) or [a.vlasjuk@t-online.de](mailto:a.vlasjuk@t-online.de) works for me :)