Fix exception thrown using MishActivation
Author: chinoll
Pull request: yes

# What does this PR do?
Fix an exception when calling MishActivation module when using pytorch 1.9.0
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Fixes [#42](https://github.com/microsoft/GLIP/issues/51)

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

What exception are we fixing? The [PyTorch documentation](https://pytorch.org/docs/1.9.0/generated/torch.nn.functional.mish.html?highlight=mish#torch.nn.functional.mish) indicates the mish function was in 1.9.0 and I just tried on a fresh env, the function can be imported without any problem on 1.9.0.

> What exception are we fixing? The [PyTorch documentation](https://pytorch.org/docs/1.9.0/generated/torch.nn.functional.mish.html?highlight=mish#torch.nn.functional.mish) indicates the mish function was in 1.9.0 and I just tried on a fresh env, the function can be imported without any problem on 1.9.0.

I am using pytorch version 1.9.0a0+c3d40fd, and using nn.functional.mish will throw AttributeError exception.