[WIP] Rewrite ProphetNet to adapt converting ONNX friendly
# What does this PR do?
We want to convert ProphetNet (pytorch model) to ONNX, but it needs some source code change to adapt it. The current code cannot convert to ONNX because 
(1) The current pytorch model generates very large TorchScript IR graph (38k for decoder). We rewrite the way it generates bias: ~~Let's use numpy and then formulate the torch tensor finally.~~
Numpy way can help convert ONNX via tracing, but we prefer using scripting here.
Add script decorator so that the model can be converted via scripting. This reduces IR graph to 5k for decoder.
(2) `torch.new` generates constant dimension for Tensor in IR graph, which is not suitable if we want to do dynamic input axes for the converter. So we use `torch.full` instead.
This PR does not (should not) change any model behavior.

Fixes # (issue)
After this PR, the model can be converted to ONNX via scripting.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?
@qiweizhen @patrickvonplaten @Zhylkaaa 
@mfuntowicz - could you take a look maybe? :-) <|||||>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/master/CONTRIBUTING.md) are likely to be ignored.