It is an EASA requirement that all flight training must take place within an ATO except for some of the CBIR, which then has to be validated by further training at an ATO, and, until the next amendment to Part-FCL, differences training. The IMC rating, is not an EASA qualification and so is not subject to the requirement.
Whopity's essential point is correct - training for the night rating may not be done out with an ATO.