Create an aggregatetuple without in model key

[maeldebon]

Original issue from the substra repository

It is possible to create an aggregatetuple without passing any --in-model-key in the command.

Issue in the backend

The in_models_keys field of an aggregatetuple isn't required and has a minimum length of 0.

As the sole purpose of the aggregatetuple is to aggregate at least 2 models together, the in_models_keys should be required and should have a minimum length of 2.

[jmorel]

@maeldebon following the short discussion we've had in the PR, I'm moving this issue to the backend repository.

[samlesu]

As said in the backend PR, there is no reason for adding this restriction. Aggregating nothing or aggregating a single model should be possible.