Add support to export Blenderbot models to onnx
# 🚀 Feature request
Add support to export Blenderbot models to onnx. Please! Or help guide towards a correct direction!

## Motivation
There is someone working to implement Blenderbot 2.0 into transformers [12807](https://github.com/huggingface/transformers/issues/12807#issuecomment-1039315898), yet we still do not have a proper 1.0 export to onnx. 

## Your contribution
### Error with torch.onnx.export
Tried to export Blenderbot 1.0 model via ```torch.onnx.export(model, input_values, output_path)``` and it failed with an error:
```ValueError: You have to specify either decoder_input_ids or decoder_inputs_embeds```
I've seen issues like this around with seq2seq models being encoder and decoder at the same time, thus needing to somehow export to models. Seems that onnx export of Bart model being of similar type has been properly implemeneted, as there's an onnx [config object](https://github.com/huggingface/transformers/blob/master/src/transformers/models/bart/configuration_bart.py).

### Weird outcome with transformers.onnx.export
Next thing I tried was to follow T5 model export implementation using Bart config as an example. I am not experienced enough in this, so obviously I stumbled onto a weird outcome which I cannot grasp / google out what's happening and why. I created a ```BlenderbotOnnxConfig(OnnxSeq2SeqConfigWithPast)``` class using simply same properties as in ```BartOnnxConfig(OnnxSeq2SeqConfigWithPast)```.
It then succesfully save files to a designated directory, but there's hundreds of them!!! Seems as if each layer is exported with its bias files separate...
When trying to do ```onnx.load``` console cries out the error about ```final_logits_bias``` file being missing. After digging into it, seems this file is in mentioned in a variable ```_keys_to_ignore_on_load_missing```, but still seems getting included. Not sure what I missed and where to go on from, therefore writing this issue for help / support.

### Notebook
You can [see the notebook](https://www.kaggle.com/danieliusv/blenderbot1-0-test-and-export) with my unsuccessful tries.


>**Motivation**
There is someone working to implement Blenderbot 2.0 into transformers https://github.com/huggingface/transformers/issues/12807#issuecomment-1039315898, yet we still do not have a proper 1.0 export to onnx.

Nice motivation! More seriously though, note that onnx support is optional and is not a core feature. There will always be some models that don't have onnx support and we usually tend to add it only when some users request it.

Also pinging our onnx expert @lewtun :) <|||||>Sorry, didn't really know what to write in the *Motivation* section... uh.. "we need it for a project"? ¯\_(ツ)_/¯
Just to be clear, we'd much rather use Blenderbot 2.0, but since it's not yet implemented, not reasonable to ask.<|||||>Hey @Darth-Carrotpie thanks for raising this issue and sharing a solid first stab at the export! I'll take a look at this later in the week and see what the problem is :)<|||||>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.<|||||>Re-open this issue