code: def _dirichlet_sparse(r, pos, f, batch_idx, aggr):
      variances = _variance_estimator_sparse(r, pos, f, batch_idx)
      return 1 / 2.0 * aggr(variances)
  
file_path: torch-points3d/torch_points3d/core/losses/dirichlet_loss.py
imports: import torch
  import torch_points_kernels
  from torch_cluster import radius
  from torch_scatter import scatter_add
label_desc: Use image node_id 4 for calling a global function with example usage: _dirichlet_sparse(r, pos, f, batch_idx, aggr) and returns: unknown