One day almost 17 years ago, Dan Mitrione, an American police adviser, was kidnapped by Tupamaro guerrillas as he left his house in suburban Carrasco. Ten days later, his body was found in an abandoned car. The kidnapping and killing became the basis for the Costa-Gavras film ''State of Siege,'' which glorified the guerrilla movement and implied that the United States Government, through its public safety assistance program, was helping to train Latin American police forces in methods of torture. Today, the man who led the Tupamaros says the guerrillas had not planned to kill Mr. Mitrione but that it had occurred because of a breakdown in communications after Uruguayan security forces captured the leaders, who were unable to send instructions to those holding him. Bullet Took Off Part of Tongue The former guerrilla chieftain, Raul Sendic, is now 62 years old and has white hair and a beard, which helps to cover the scar where a bullet passed through his cheek and took o...