Is there bathrooms outside the forum, for people that are waiting for the concert?

The bathrooms are inside but accessible. If you are waiting in the immediate area outside surrounding the foyer.

Sometimes i have seen those Porta Pitt for different events

Yes

There are.. it only at certain points.

If you are in line waiting to pass through security then no, you cannot access the bathrooms. However after you go through security there are bathrooms you can use anytime.so there ARE bathrooms "outside" of the actual concert but they are still inside the building and the line to get into the building is outside the building's perimiter so you can only access them after you pass through security

Sometimes you can fond porta poties.