Decorates a builder of model class.
The builder should be a Callable (a class or a function).
This decorator supports registration of backbone builder as follows:
def builder(input_specs, config, l2_reg):
# Builds a MyModel object.
my_backbone = build_backbone_3d(input_specs, config, l2_reg)
the key to look up the builder.
A callable for use as class decorator that registers the decorated class
for creation from an instance of model class.