fv_variations_unit
==================

.. py:function:: t3toolbox.backend.fv_operations.fv_variations_unit(variation_shapes, index, stack_shape = (), use_jax = False)

   .. code-block:: python

      def fv_variations_unit(
              variation_shapes:   VariationShapes,
              index:              typ.Tuple[bool, int, typ.Sequence[int]],  # (use_tt_coordinate, i, within_index)
              stack_shape:        typ.Sequence[int] = (),
              use_jax:            bool = False,
      ) -> Variations:


   Canonical unit variation: all-zero cores except a single entry set to 1.

   ``index = (use_tt_coordinate, i, within_index)`` selects the family (tt if ``use_tt_coordinate``
   else tucker), the core position ``i``, and the within-core entry (broadcast over the stack).