I'm not sure how much Amazon have actually contributed to MXNet, delving into some background on it seems to suggest it was driven by the University of Washington and Carnegie Mellon, with Amazon coming to the conclusion it was pretty cool tech and promoting it on AWS.
I'd like to be proved wrong on this front as it'd be very nice to see Amazon start to step up and give something back to the open source community, where they seriously lag behind the likes of Google, Facebook, Twitter, and even <cough> Microsoft