Batch Norm should allow for a functional API

[albertz]

Functional meaning that the parameters are arguments.

[albertz]

Note that with param_version=2, as the params are all vectors, there is no way really to get it wrong via reuse_params and layer_output, so while maybe slightly ugly, with reuse_params and layer_output this already exists.

[albertz]

(I totally forgot that the user can also provide shape (list of Dims) to reuse_params to make it explicit anyway. Although not needed here.)

[albertz]

I think this can be closed now, as we also have an implementation on returnn-common side now, unless there is some real demand for this later.