Fix issues with forwardDims()

To be merged after !90 (merged).

Fixes two issues with forwardDims():

• The previous iterative algorithm may be stuck in specific cases (see example below);
• It should always recompute all nodes output dims, because the graph may have changed.

Example of stuck case:

1. Initial graph is output dims forwarded;
2. Scaling operators are inserted before some Conv;
3. The graph is output dims re-forwarded. Here is what might happen:
   • Start the forward dims list with Producers;
   • Move to next child nodes: the Conv;
   • Conv cannot be forward dim because their new input, a Scaling node, is not forward dim;
   • Check if Conv childs are forward dim: if they are not directly the new Scaling node, they are!
   • We get stuck in a state where listNodes == nextList, although all nodes output dim are computable!