Remove mentions to `async`reserved keyword for usage with Python 3.7

[douglasrizzo]

In this PR, I've deleted all mentions to the async keyword in the master branch of ELF to resume compatibility of the repository with Python 3.7

ELF uses async as a name for variables and function parameters. In Python 3.7, async has become a reserved word. The word is only used as argument to the copy_ and cuda methods, which are methods from the pytorch.Tensor object. But these methods don't use the value of the async argument for anything.

By removing mentions to the now-reserved keyword, no change in the behavior of the package is expected. I've tested training a model using ./train_minirts.sh on Python 3.7 with both CPU and GPU and everything worked. I believe this fixes #124 and #127.

I created a new PR because I used my master branch as the head repository in #134, which was a bad idea on my side.

[carlosgmartin]

Any updates on this?

[douglasrizzo]

I don't think this project is still active. If you want to use this PR, you can fork the repo and apply the PR to your fork. Or just clone my fork, where I fixed this.