Always enable flat net construction

[albertz]

Fix #992.

Maybe also fix #957 (need to test this).

[albertz]

The subnet issue described in #957 is maybe like this:

Root net has some CondLayer or MaskedComputationLayer or RecLayer, which itself goes back to the root net.

_NetworkConstructionStack is actually per net but in flat_construct, it catches all _DelayedConstructionException, even for other nets. This is probably wrong.

[albertz]

About other exceptions in construct_layer: We placed the flat construction logic later because we want that LayerNotFound is directly raised. We maybe should generalize this. we can effectively catch any raised exceptions, and once construct_layer is called again, it would reraise that earlier exception. Or when called again, then not run into the flat construction but directly construct? Maybe allow construction stack depth 2?

[albertz]

Need some comment why nested flat construction is ok. Basically: E.g. in RecLayer, when getting back to base net, we don't want a partial rec template construction to reset. So, it will not be deep w.r.t. to such layers which define new ctx boundaries. But that is not really a problem.

[albertz]

The handling of exceptions is problematic, esp together with the Rec template construction.