What is the ruling on visiting some of the graves in Medina, such as Al-Baqi and the martyrs of Uhud?

Visiting graves is a Sunnah everywhere, including visiting Al-Baqi' and the martyrs of Uhud, and visiting Quba Mosque to pray therein. As for other mosques, such as the Seven Mosques and Al-Ghamamah Mosque, there is no basis for visiting them with the intention of worship, and it is considered an innovation (bid'ah).