code: def partial_transform(self, traj):
          
          if self.atom_indices is not None:
              sliced_traj = traj.atom_slice(self.atom_indices)
          else:
              sliced_traj = traj
          result = libdistance.cdist(
              sliced_traj, self.sliced_reference_traj, 'rmsd'
          )
          return self._transform(result)
docs: Featurize an MD trajectory into a vector space via distance
          after superposition
  
          Parameters
          ----------
          traj : mdtraj.Trajectory
              A molecular dynamics trajectory to featurize.
  
          Returns
          -------
          features : np.ndarray, shape=(n_frames, n_ref_frames)
              The RMSD value of each frame of the input trajectory to be
              featurized versus each frame in the reference trajectory. The
              number of features is the number of reference frames.
  
          See Also
          --------
          transform : simultaneously featurize a collection of MD trajectories