Fixing OPT fast tokenizer option.
Author: Narsil
Pull request: yes

# What does this PR do?

Fixes the relevant issues: 

https://huggingface.co/wjmcat/opt-350m-paddle/discussions/1
https://huggingface.slack.com/archives/C01N44FJDHT/p1653511495183519 (internal link)
https://github.com/huggingface/transformers/pull/17088#discussion_r871246439

Basically, the OPT tokenizer is a GPT2 tokenizer that adds a BOS token at the start
of the tokens.
Lots of back&forth at the time, but the truth is that the `ByteLevel(trim_offsets=False)` 
post_processor, actually doesn't do anything, so we can just replace it with a simple
`TemplateProcessing` processor and everything works correctly.

This PR fixes the biggest culprit (missing BOS token on the fast tokenizer version).

Call to witness on other issues I might have missed.

@SauLu
@patrickvonplaten
@mishig

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Fixes # (issue)


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Good point! 

Indeed, I had not noticed at all that `ByteLevel(trim_offsets=False)` was not doing anything. As `trim_offset` is not an argument in the `__init__` either, I don't see any problem that your solution could cause! :blush: