As pointed out half a year ago, it is important to distinguish between the FAA system and something foreign. As the OP said ATPL, they are probably talking about foreign licensing. Under the FAA system there is no requirement for ground school prior to taking the FAA ATP written exam. There is no requirement for an instructor's sign off prior to taking the written. If you are taking the ATM written there is a requirement to complete the CTP class but that class has next to nothing to do with what is covered on the ATM written and will not prepare you to take the ATM. If you are taking the checkride in a light airplane there is no requirement for training or an instructor sign off prior to the checkride. The FAA thinks if you are at the level of getting an ATP you should know if you are ready for the written and checkride.

If you are getting a type rating along with the ATP (or adding a type to any level certificate) in a simulator it falls under Part 142 and there is a requirement for formal aircraft ground school and flight training and a sign off.