GatherLayer resolved wrong common axes

[albertz]

The code in _get_common_input_position_axes used dict(allow_same_spatial_dim=True). For some different static dim tags which happen to have the same static dim value, this would see them as equal, which is wrong in general.

We could introduce a new behavior version to be more restrict. But this might make it difficult for some people to update?

Or only match auto-generated same spatial/feature dims but never user-generated?

Very related is also #1154 and #865.