Out dim tags potentially needs duplicated logic

[albertz]

#17 (comment) lead us to have out_spatial_dim as an optional arg to many functions and modules, e.g. Conv.

When this is not passed (also see #115), then some default dim tag is created. When this is a dynamic dim, this is fine and no extra logic is needed. When this is static, we need to know the resulting dim in advance, and basically need to duplicate the logic from RETURNN, e.g. ConvLayer.calc_out_dim.

This issue is also on pure RETURNN side, and reported here: rwth-i6/returnn#955

However, we probably need to have some changes here on returnn-common side as well.

[albertz]

rwth-i6/returnn#955 is fixed now via rwth-i6/returnn#974.

I think this solves the issue already here.