code: def cp_lstsq_grad(cp_tensor, tensor, return_loss=False, mask=None):
      """This function computes (for a third-order tensor)
  
      .. math::
  
          \nabla 0.5 ||\\mathcal{X} - [\\mathbf{w}; \\mathbf{A}, \\mathbf{B}, \\mathbf{C}]||^2
  
      where :math:`[\\mathbf{w}; \\mathbf{A}, \\mathbf{B}, \\mathbf{C}]` is the CP decomposition with weights
      :math:`\\mathbf{w}` and factor matrices :math:`\\mathbf{A}`, :math:`\\mathbf{B}` and :math:`\\mathbf{C}`.
  
      Note that this does not return the gradient with respect to the weights even if CP is normalized.
  
      Parameters
      ----------
      cp_tensor : CPTensor = (weight, factors)
          factors is a list of factor matrices, all with the same number of columns
          i.e. for all matrix U in factor_matrices:
          U has shape ``(s_i, R)``, where R is fixed and s_i varies with i
  
      mask : ndarray
          A mask to be applied to the final tensor. It should be
          broadcastable to the shape of the final tensor, that is
          ``(U[1].shape[0], ... U[-1].shape[0])``.
  
      return_loss : bool
          Optionally return the scalar loss function along with the gradient.
  
      Returns
      -------
      cp_gradient : CPTensor = (None, factors)
          factors is a list of factor matrix gradients, all with the same number of columns
          i.e. for all matrix U in factor_matrices:
          U has shape ``(s_i, R)``, where R is fixed and s_i varies with i
  
      loss : float
          Scalar quantity of the loss function corresponding to cp_gradient. Only returned
          if return_loss = True.
      """
      _validate_cp_tensor(cp_tensor)
      _, factors = cp_tensor
  
      diff = tensor - cp_to_tensor(cp_tensor)
  
      if mask is not None:
          diff = diff * mask
  
      grad_fac = [
          -unfolding_dot_khatri_rao(diff, cp_tensor, ii)
          for ii in range(len(factors))
      ]
  
      if return_loss:
          return CPTensor((None, grad_fac)), 0.5 * T.sum(diff**2)
  
      return CPTensor((None, grad_fac))
  
imports: from .None import backend
  from .base import fold, tensor_to_vec
  from ._factorized_tensor import FactorizedTensor
  from .tenalg import khatri_rao, unfolding_dot_khatri_rao
  from .metrics.factors import congruence_coefficient
  import numpy
label_desc: Use image node_id 6 for calling a global function with example usage: cp_lstsq_grad(cp_tensor, tensor, return_loss, mask) and returns: CPTensor, CPTensor,  unknown