Although Uighur is likely, I doubt sponsor or training. First this is terrorist in nature, but not necessarily foreign. The Uighur nationality has a long grudge against ethnic Chinese, and ethnic Chinese have visa versa so domestic seems more likely.
second if you compared the actions in China to even the attacks in neighboring countries with strict gun control like Indian you, would note the choice of arms in Chinese attacks are always knives, clubs and hand to hand weapons. The Indian attack in Mumbai used AK's the attacks in Pakistan use explosives and AK's. If these attackers in China could have left the country and gotten foreign terrorist training then why not bomb? I mean terrorist groups know how to make homemade explosives and detonators, and if they could sneak out of the country with ease to attend such terrorists camps then why not establish a gun running route? I mean the casualty rate would be much higher. Hence my doubt of foreign sponsoring or training.
I do feel sorrow for the family's of victims.
This could also be a Tibetan terrorist group.
But I agree Uighur is more likely. The sort of attack against civilians far outside of own territory is a form of terrorism that is relatively new and modern, and not traditional to Uighur or anyone else. While Islamic fundamentalist groups are not the only practitioners of this sort of terrorism, they are certainly by far the most prolific and well known practitioners of this. It certainly seem likely that if it were carred out by uighurs, then the methodology, and possibly finer points of ideology beyond mere anti-Chinese ethnic resentment, is inspired from abroad.