Fix/low rank conversion pointcloud

src/ott/geometry/pointcloud.py

@@ -278,19 +278,27 @@ def to_LRCGeometry(
         the point cloud does not have squared Euclidean cost.
 
     Returns:
-      Returns the unmodified point cloud if :math:`n m \ge (n + m) d`, where
-      :math:`n, m` is the shape and :math:`d` is the dimension of the point
-      cloud with squared Euclidean cost.
-      Otherwise, returns the re-scaled low-rank geometry.
+      :class:`~ott.geometry.low_rank.LRCGeometry` or
+      :class:`~ott.geometry.pointcloud.PointCloud`:
+      The re-scaled low-rank geometry if:
+
+      1. :math:`n m > (n + m) d`, where
+         :math:`n, m` is the shape and :math:`d` is the
+         dimension of the point cloud with squared
+         Euclidean cost; or
+      2. ``rank=0`` is passed as keyword argument.
+
+      Otherwise, returns the unmodified point cloud.
     """
+    force = (kwargs.get("rank") == 0)
     if self.is_squared_euclidean:
-      if self._check_LRC_dim:
+      if force or self._check_LRC_dim:
         return self._sqeucl_to_lr(scale)
       # we don't update the `scale_factor` because in GW, the linear cost
       # is first materialized and then scaled by `fused_penalty` afterwards
       return self
     if self.is_neg_dotp:
-      if self._check_LRC_dim:
+      if force or self._check_LRC_dim:
         return self._dotp_to_lr(scale)
       return self
     return super().to_LRCGeometry(scale=scale, **kwargs)