Update modeling_longformer.py so it can be converted to onnx format using Pytorch.

[xiaowuhu]

What does this PR do?

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, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? 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, and here are tips on formatting docstrings. It don't need to change document.
• [ ] Did you write any new necessary tests? No necessary test need because it is a tiny fix.

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.