Blenderbot-3B config seems to be a little wrong
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 4.1
- Platform: Linux
- Python version: 3.8
- PyTorch version (GPU?):- 
- Tensorflow version (GPU?): - 
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

It seems the current Config of `Blenderbot-3B` is a bit broken, (`Blenderbot-90M` and distill versions seem fine).

```python

tokenizer = AutoTokenizer.from_pretrained('facebook/blenderbot-90M')
tokenizer.decode(tokenizer.encode("Hey there"))
# 'hey there'  so working fine

tokenizer.decode(tokenizer.encode("Hey there"))
# '<unk> y <unk> e'   obvious error as the tokens as 'ĠHey' exists in the vocab. Error is possibly linked to '@@' string terminator config

----
# Other example that's probably linked but that originally triggered the issue so we need to make sure it's fixed too

nlp = pipeline('text-generation', model='blenderbot-3B')
nlp("Hey there")
# {"generated_text": "'ĠHi, Ġhow Ġare Ġyou Ġtoday? ĠI Ġjust Ġgot Ġback Ġfrom Ġa Ġwalk, Ġit Ġwas Ġnice."}

```

### Who can help
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@patrickvonplaten @patil-suraj 

## Information

Model I am using (Bert, XLNet ...):

The problem arises when using:
* [x ] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## Expected behavior

The tokenization should be better at encoding for 3B. And the pipeline should not output garbage Ġ everywhere.

<!-- A clear and concise description of what you would expect to happen. -->

This issue has been stale for 1 month.<|||||>Closing this, blenderbot 90M is very different in Arch as other variants, so it will receive less love (it's not that powerful compared to the others anyway).

Also a lot of work was done here : https://github.com/huggingface/transformers/pull/10002