Update modeling_longformer.py so it can be converted to onnx format using Pytorch.
Author: xiaowuhu
Pull request: yes

# What does this PR do?

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In longformer model,  the original 'is_global_attn=None' can works, but when using torch.onnx.export() it will be failed when meet the below line `if is_global_attn:` 
So I need to change it to explicitly checking like 'if is_global_attn == True:', also set the default value as False in signature 'is_global_attn=False'.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
      No, it is tiny code fix.
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
      No related issue. I try to convert the model to onnx but failed, so make some tiny change.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
  It don't need to change document.
- [ ] Did you write any new necessary tests?
 No necessary test need because it is a tiny fix.


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