Better way to specify axis for layers operating on rec time axis

[Zettelkasten]

This is a special case of #597.

I noticed that for #776 in WindowLayer, but also for e.g. CumsumLayer that operate on the rec time axis, we have a check like this:

    if axis == "T" and data.time_dim_axis is None:
      # Assume inside RecLayer.
      # (then the code for a single step)
    else:
      axis = data.get_axis_from_description(axis)
      # (then the code for all steps at once, operating on `axis`)

This is flawed as soon as the input has two dynamic dims: Then the input might have a time_dim_axis even in the recurrent case, and the layer will silently behave very wrongly.

Instead, I would propose to allow axis to be a DimensionTag. Then, we can just check axis == network.get_rec_parent_layer(inside_loop=False).time_dim_tag.
Would we also want to allow the stag:description syntax here?
Also, in #391 for CumConcatLayer, we briefly thought of allowing :i as axis name. Maybe allow this too?

I worked on this a month ago, for CumsumLayer, see #780. But I never tested this, it was just something a colleague of mine stumbled across but then gave up on. Now with #776, and dim tags becoming popular, I remembered to raise this issue here.

[albertz]

Yes, I agree that this should be done better. See also my comments on your PR. I think with consistent dim tags, we can simply do the check like axis == network.get_inside_rec_time_dim(). This should be exactly the recurrent case (when axis is a DimensionTag).