Replace async with non_blocking for Python 3.7

[cclauss]

async is now a reserved word in Python >= 3.7 so use non_blocking instead. pytorch/pytorch#4999

flake8 testing of https://github.com/quark0/darts on Python 3.7.0

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

./cnn/train.py:119:40: E999 SyntaxError: invalid syntax
    target = Variable(target).cuda(async=True)
                                       ^
./cnn/train_imagenet.py:176:30: E999 SyntaxError: invalid syntax
    target = target.cuda(async=True)
                             ^
./cnn/train_search.py:145:61: E999 SyntaxError: invalid syntax
    target = Variable(target, requires_grad=False).cuda(async=True)
                                                            ^
3     E999 SyntaxError: invalid syntax
3

[alphadl]

Thx, it works for me~