fv_variation_shapes
===================

.. py:function:: t3toolbox.backend.fv_operations.fv_variation_shapes(frame)

   .. code-block:: python

      def fv_variation_shapes(
              frame:  typ.Tuple,   # (U, O, P, Q) frame data; len=d each, elm_shape=stack_shape+(...)
      ) -> VariationShapes:        # (tucker_variation_shapes, tt_variation_shapes); WITHOUT the stack


   The variation shapes of a frame: ``(nD_i, N_i)`` per Tucker variation and
   ``(rL_i, nU_i, rR_i)`` per TT variation, read off the frame cores.

   The backend twin of the frontend ``T3Frame.variation_shapes`` -- and the ONE place the
   frame-to-variation axis convention is written down for the ragged layer. Frame bookkeeping is
   non-obvious (the TT variation takes its left bond from the core itself but its right bond from the
   core's own trailing axis, which is the NEXT bond -- the gauge shift ``left[:-1]`` / ``right[1:]``
   in mask form), so open-coding these indices is how they drift. Feed the result to
   :py:func:`fv_variations_zeros` / :py:func:`fv_variations_randn`.