Shift from 'async' to 'non_blocking' in cuda

[cclauss]

async is a reserved word in Python 3.7 so cuda is deprecating the function parameter async in favor of __non_blocking__.

https://github.com/pytorch/pytorch/blob/4d28b65fb8978f1c34765d56f18c07042e70a43a/torch/_utils.py#L20-L21

flake8 testing of https://github.com/facebookresearch/ParlAI on Python 3.7.0

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./projects/personachat/persona_seq2seq.py:2220:43: E999 SyntaxError: invalid syntax
                    self.zs.copy_(zs, async=True)
                                          ^
./parlai/agents/vsepp_caption/vsepp_caption.py:160:38: E999 SyntaxError: invalid syntax
            images = images.cuda(async=True)
                                     ^
./parlai/agents/drqa/model.py:89:43: E999 SyntaxError: invalid syntax
            inputs = [Variable(e.cuda(async=True)) for e in ex[:5]]
                                          ^
3     E999 SyntaxError: invalid syntax
3

[Redredperfect]

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