Update how loss function work

Add clearer error message for missing input gradient for FC.

This MR aims at fixing this issue following the solution described in: aidge_learning#1 (closed)

changed milestone to %aidge - April 2024

requested review from @pineapple, @thibaultallenet, and @olivierbichler

assigned to @cmoineau

mentioned in commit be217da2

merged

mentioned in merge request !93 (merged)