Nope only after getting ECOPR, until you get your ECOPR you will be stuck in Canada because in P2 they are going to ask you to declare that if you are in Canada or not, but If you have Canadian Visa then you can go to USA and come back but once you get your P2 you will need to come back to Canada to declare that you are in Canada and ready to get your Confirmation of PR so I would suggest you to wait until you get ECOPR before leaving but if it's urgent then you can leave but make sure to come back once you get P2 and also make sure you have Canadian Visa so that you can enter again.