Make Transformer tolerate missing layers for PP

[wconstab]

Stack from ghstack (oldest at bottom):

• #318
• -> #322
• #321

A few small changes here lets manual PP frontend 'reconfigure' a whole transformer model to a stage's portion simply by setting undesired layers to None (in cases of top level layers) or deleting them from the ModuleDict (for 'layers.*').

These changes don't impact the FQNs of the remaining layers, which is critical for checkpoint load/save compatibility.