Just for the record: I see that my reply wasn't well formulated, as it's perfectly correct to refer to a torpedo like this as a "practicetorpedo" (in german "übungstorpedo" versus "gefechtstorpedo").
I just wanted to point out that the there were no dedicated torpedoes for exercises, as the same torpedo would/could be used for exersice-shots numerous times until it eventually was used for a "live" launch with a warhead.
During the course of the war, the germans introduced a special version of the G7a(TI) though: The G7a(TIü), which was only used for the initial training of uboat-crews. It had minor modifications to reduce weight, a special variant of the TA I depthmechanism and was equipped with a light (ie not waterfilled) exersicehead. These measures ensured that the torpedo had positive buoyancy at the end of the run, and that the engine were lessed stressed (= less wear) compared to the original G7a.
The point was to get a higher number of exersice-shots and reduce loss of torpedoes, hence expedite and improve efficiency of training.