code: def backward(
      self, dnrows, dncols, dC_indptr, dC_indices, dC_eids, dC_weights
  ):
      # Only the last argument is meaningful.
      gidxC = self.backward_cache
      return tuple(
          csrmask(gidxC, dC_weights, gidx) for gidx in self.gidxs
      )
  
imports: import mxnet
  import numpy
  from mxnet import nd
  from ..._sparse_ops import _bwd_segment_cmp, _csrmask, _csrmm, _csrsum, _gsddmm, _gspmm, _scatter_add, _segment_reduce
  from ...base import ALL, dgl_warning, is_all
  from ...heterograph_index import create_unitgraph_from_csr
  from .tensor import asnumpy, context, copy_to, to_backend_ctx, zerocopy_from_numpy
label_desc: Use image node_id 3 for calling the CSRSum obj's underlying member method code with example usage: obj.backward(dnrows, dncols, dC_indptr, dC_indices, dC_eids, dC_weights) and returns: tuple