's season 21 Mega Mentor is… Ed Sheeran!Following in the footsteps of musical pals like Taylor Swift and Miley Cyrus, Sheeran is joining the NBC singing competition to help coaches Kelly Clarkson, Ariana Grande, John Legend and Blake Shelton guide their singers through the upcoming Knockout Rounds, the final step before the competition moves to the fan-voted live shows.Sheeran will coach all the teams equally, while the previously announced season 21 celebrity Battle Round advisors are specific