So both 172 and 176 are training units? what about 171 and 177, also training units?
I wouldn't say the 172nd and 176th are basic training units per se where pilots first learn to fly the the J-20. If we were to draw parallels to the USAF, I'd say that the 172nd in Cangzhou is more of a Weapons Squadron at the Fighter Weapons School in Nellis Air Force Base - they train crew on the application of advanced tactics. The 176th in Dingxin on the other hand are like Test and Evaluation Squadrons (also at Nellis Air Force Base) or a Flight Test Squadron (at Edwards Air Force Base), where they test out new avionics/weapons/components and develop the advanced tactics to be applied across the PLAAF. In other words pilots show up to the 172nd and 176th already qualified (and probably quite experienced and seasoned) on the J-20.
AFAIK the 171st is a J-10 and J-11 unit that does the same role as the 172nd does for the J-20 and J-16 at Cangzhou, while the 177th is an aggressor unit (NATO/US calls it red air, we call it blue air) that also flies J-10s and J-11s. The PLAAF doesn't exactly have training units along the lines of the USAF's Formal Training Units or the USN's Replacement Air Group - conversion training is conducted at the units pilot/WSOs are assigned to once they've graduated from Air University and the respective PLAAF Flying Academies.