code: def _phi_for_cholesky_(A):
      "Modifies A to be the phi function used in differentiating through Cholesky"
      A.tril_().diagonal(offset=0, dim1=-2, dim2=-1).mul_(0.5)
      return A
  
imports: import abc
  import torch
  from linear_operator.operators import CholLinearOperator, TriangularLinearOperator
  from linear_operator.utils.cholesky import psd_safe_cholesky
  from ..distributions import MultivariateNormal
  from ._variational_distribution import _VariationalDistribution
label_desc: Use image node_id 2 for calling a global function with example usage: _phi_for_cholesky_(A) and returns: A