'async' is a reserved word in Python >= 3.7

Same problem and recommended fix as CSAILVision/semantic-segmentation-pytorch#125

[flake8](http://flake8.pycqa.org) testing of https://github.com/CSAILVision/unifiedparsing on Python 3.7.1

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./lib/nn/parallel/data_parallel.py:16:31: E999 SyntaxError: invalid syntax
v = obj.cuda(dev, async=True)
^
./models/models.py:163:27: F821 undefined name 'ResnetDilated'
net_encoder = ResnetDilated(orig_resnet,
^
./models/models.py:168:27: F821 undefined name 'ResnetDilated'
net_encoder = ResnetDilated(orig_resnet,
^
1 E999 SyntaxError: invalid syntax
2 F821 undefined name 'ResnetDilated'
3
```