AIIMS Patna Vs AIIMS Nagpur which one should I pick in USMLE prep point of view? Are there students in AIIMS Patna preparing for USMLE?

It really does not matter which institute you do your medical graduation from until and unless you become an international medical graduate. That is all that US officials care about. 

• Although, colleges like AIIMS Delhi, Manipal, CMC Vellore, MAMC, JIPMER have the perk of having a very good network that might help you in getting more interviews. 
• AIIMS Patna or AIIMS Nagpur can not help you build a network as of now since they are just newly established. 
• If you are willing to pick from the perspective of having students around you preparing for USMLE too, then it is not much of a problem as every college has some students like that. 
• Do keep in mind that you will have to prepare on your own and you can always ask professors if you have any doubts. 

You can also opt for getting into state colleges like KGMU or Seth GSMC where you can be surrounded by other USMLE aspirants.