cuda(async) —> cuda(non_blocking) for Python >= 3.7
cuda(async) —> cuda(non_blocking) for Python >= 3.7

Fixes #125 

__async__ is a reserved word in Python 3.7 and later.  To fix this pytorch/pytorch#4999 changed __cuda(async=True)__ to __cuda(non_blocking=True)__ so this PR tracks with that change which landed in PyTourch 0.4.1.
hangzhaomit/semantic-segmentation-pytorch,hangzhaomit/semantic-segmentation-pytorch