[Feature] Support aggregate multiple edge features in to_simple. (#2623)

* upd

* fix

* upd

* upd

* upd

* upd

* upd

* upd

* upd

* upd

* upd

* fix

* refactor

* upd test

* large feat_len or n in segment reduce

* lint

python/dgl/backend/backend.py

@@ -1552,6 +1552,28 @@ def segment_reduce(op, x, offsets):
     """
     pass
 
+def scatter_add(x, idx, m):
+
+    """Scatter add (on first dimension) operator.
+
+    Math: y[idx[i], *] += x[i, *]
+
+    Parameters
+    ----------
+    x : Tensor
+        The input feature.
+    idx : Tensor
+        The indices array.
+    m : int
+        The length of output.
+    
+    Returns
+    -------
+    Tensor
+        The output tensor.
+    """
+    pass
+
 
 ###############################################################################
 # Other interfaces

python/dgl/backend/mxnet/sparse.py

 import mxnet as mx
 import numpy as np
 from mxnet import nd
-from ...sparse import _gspmm, _gsddmm, _segment_reduce, _bwd_segment_cmp
+from ...sparse import _gspmm, _gsddmm, _segment_reduce, _bwd_segment_cmp, _scatter_add
 from ...base import dgl_warning, is_all, ALL
 from .tensor import asnumpy, copy_to, zerocopy_from_numpy, context, to_backend_ctx
 
-__all__ = ['gspmm', 'gsddmm', 'edge_softmax', 'segment_reduce']
+__all__ = ['gspmm', 'gsddmm', 'edge_softmax', 'segment_reduce', 'scatter_add']
 
 
 def _scatter_nd(index, src, n_rows):
@@ -360,3 +360,22 @@ class SegmentReduce(mx.autograd.Function):
 def segment_reduce(op, x, offsets):
     segment_reduce_op = SegmentReduce(op, offsets)
     return segment_reduce_op(x)
+
+
+class ScatterAdd(mx.autograd.Function):
+    def __init__(self, idx, m):
+        super(ScatterAdd, self).__init__()
+        self.idx = idx
+        self.m = m
+
+    def forward(self, x):
+        y = _scatter_add(x, self.idx, self.m)
+        return y
+    
+    def backward(self, dy):
+        return dy[self.idx]
+
+
+def scatter_add(x, idx, m):
+    scatter_add_op = ScatterAdd(idx, m)
+    return scatter_add_op(x)

python/dgl/backend/pytorch/sparse.py

 import torch as th
 from distutils.version import LooseVersion
 from ...base import is_all, ALL
-from ...sparse import _gspmm, _gsddmm, _segment_reduce, _bwd_segment_cmp
+from ...sparse import _gspmm, _gsddmm, _segment_reduce, _bwd_segment_cmp, _scatter_add
 
 if LooseVersion(th.__version__) >= LooseVersion("1.6.0"):
     from torch.cuda.amp import custom_fwd, custom_bwd
@@ -24,7 +24,7 @@ else:
             return bwd(*args, **kwargs)
         return decorate_bwd
 
-__all__ = ['gspmm', 'gsddmm', 'edge_softmax', 'segment_reduce']
+__all__ = ['gspmm', 'gsddmm', 'edge_softmax', 'segment_reduce', 'scatter_add']
 
 
 def _reduce_grad(grad, shape):
@@ -288,6 +288,21 @@ class SegmentReduce(th.autograd.Function):
         return None, dx, None
 
 
+class ScatterAdd(th.autograd.Function):
+    @staticmethod
+    @custom_fwd(cast_inputs=th.float16)
+    def forward(ctx, x, idx, m):
+        y = _scatter_add(x, idx, m)
+        ctx.save_for_backward(idx)
+        return y
+
+    @staticmethod
+    @custom_bwd
+    def backward(ctx, dy):
+        idx = ctx.saved_tensors
+        return dy[idx], None, None
+
+
 def gspmm(gidx, op, reduce_op, lhs_data, rhs_data):
     return GSpMM.apply(gidx, op, reduce_op, lhs_data, rhs_data)
 
@@ -302,3 +317,6 @@ def edge_softmax(gidx, logits, eids=ALL, norm_by='dst'):
 
 def segment_reduce(op, x, offsets):
     return SegmentReduce.apply(op, x, offsets)
+
+def scatter_add(x, idx, m):
+    return ScatterAdd.apply(x, idx, m)

python/dgl/backend/tensorflow/sparse.py

@@ -2,9 +2,9 @@ import tensorflow as tf
 import numpy as np
 from .tensor import tensor, copy_to, context, asnumpy, zerocopy_from_numpy
 from ...base import is_all, ALL
-from ...sparse import _gspmm, _gsddmm, _segment_reduce, _bwd_segment_cmp
+from ...sparse import _gspmm, _gsddmm, _segment_reduce, _bwd_segment_cmp, _scatter_add
 
-__all__ = ['gspmm', 'gsddmm', 'edge_softmax', 'segment_reduce']
+__all__ = ['gspmm', 'gsddmm', 'edge_softmax', 'segment_reduce', 'scatter_add']
 
 
 def _scatter_nd(index, src, n_rows):
@@ -279,3 +279,19 @@ def segment_reduce(op, x, offsets):
     def _lambda(x):
         return segment_reduce_real(op, x, offsets)
     return _lambda(x)
+
+
+def scatter_add_real(x, idx, m):
+    y = _scatter_add(x, idx, m)
+
+    def scatter_add_backward(dy):
+        return tf.gather(dy, idx)
+    
+    return y, scatter_add_backward
+
+
+def scatter_add(x, idx, m):
+    @tf.custom_gradient
+    def _lambda(x):
+        return scatter_add_real(x, idx, m)
+    return _lambda(x)

python/dgl/sparse.py

 """Module for sparse matrix operators."""
 # pylint: disable= invalid-name
 from __future__ import absolute_import
-
 import dgl.ndarray as nd
 from ._ffi.function import _init_api
 from .base import DGLError
@@ -309,6 +308,35 @@ def _segment_reduce(op, feat, offsets):
     return out, arg
 
 
+def _scatter_add(x, idx, m):
+    r""" Scatter add operator (on first dimension) implementation.
+
+    Math: y[idx[i], *] += x[i, *]
+
+    Parameters
+    ----------
+    x : Tensor
+        The input feature.
+    idx : Tensor
+        The indices array.
+    m : int
+        The length of output.
+
+    Returns
+    -------
+    Tensor
+        The output tensor.
+    """
+    out_shp = (m,) + F.shape(x)[1:]
+    ctx = F.context(x)
+    dtype = F.dtype(x)
+    out = F.zeros(out_shp, dtype, ctx)
+    _CAPI_DGLKernelScatterAdd(to_dgl_nd(x),
+                              to_dgl_nd(idx),
+                              to_dgl_nd_for_write(out))
+    return out
+
+
 def _bwd_segment_cmp(feat, arg, m):
     r""" Backward phase of segment reduction (for 'min'/'max' reduction).
 

python/dgl/transform.py

@@ -9,6 +9,7 @@ from ._ffi.function import _init_api
 from .base import dgl_warning, DGLError
 from . import convert
 from .heterograph import DGLHeteroGraph, DGLBlock
+from .frame import Frame
 from . import ndarray as nd
 from . import backend as F
 from . import utils, batch
@@ -1841,11 +1842,63 @@ def to_block(g, dst_nodes=None, include_dst_in_src=True):
 
     return new_graph
 
+def _coalesce_edge_frame(g, edge_maps, counts, aggregator):
+    r"""Coalesce edge features of duplicate edges via given aggregator in g.
+
+    Parameters
+    ----------
+    g : DGLGraph
+        The input graph.
+    edge_maps : List[Tensor]
+        The edge mapping corresponding to each edge type in g.
+    counts : List[Tensor]
+        The number of duplicated edges from the original graph for each edge type.
+    aggregator : str
+        Indicates how to coalesce edge features, could be ``arbitrary``, ``sum``
+        or ``mean``.
+
+    Returns
+    -------
+    List[Frame]
+        The frames corresponding to each edge type.
+    """
+    if aggregator == 'arbitrary':
+        eids = []
+        for i in range(len(g.canonical_etypes)):
+            feat_idx = F.asnumpy(edge_maps[i])
+            _, indices = np.unique(feat_idx, return_index=True)
+            eids.append(F.zerocopy_from_numpy(indices))
+
+        edge_frames = utils.extract_edge_subframes(g, eids)
+    elif aggregator in ['sum', 'mean']:
+        edge_frames = []
+        for i in range(len(g.canonical_etypes)):
+            feat_idx = edge_maps[i]
+            _, indices = np.unique(F.asnumpy(feat_idx), return_index=True)
+            _num_rows = len(indices)
+            _data = {}
+            for key, col in g._edge_frames[i]._columns.items():
+                data = col.data
+                new_data = F.scatter_add(data, feat_idx, _num_rows)
+                if aggregator == 'mean':
+                    norm = F.astype(counts[i], F.dtype(data))
+                    norm = F.reshape(norm, (F.shape(norm)[0],) + (1,) * (F.ndim(data) - 1))
+                    new_data /= norm
+                _data[key] = new_data
+
+            newf = Frame(data=_data, num_rows=_num_rows)
+            edge_frames.append(newf)
+    else:
+        raise DGLError("Aggregator {} not regonized, cannot coalesce edge feature in the "
+                       "specified way".format(aggregator))
+    return edge_frames
+
 def to_simple(g,
               return_counts='count',
               writeback_mapping=False,
               copy_ndata=True,
-              copy_edata=False):
+              copy_edata=False,
+              aggregator='arbitrary'):
     r"""Convert a graph to a simple graph without parallel edges and return.
 
     For a heterogeneous graph with multiple edge types, DGL treats edges with the same
@@ -1886,12 +1939,19 @@ def to_simple(g,
     copy_edata: bool, optional
         If True, the edge features of the simple graph are copied
         from the original graph. If there exists duplicate edges between
-        two nodes (u, v), the feature of the edge is randomly selected
-        from one of the duplicate edges.
+        two nodes (u, v), the feature of the edge is the aggregation
+        of edge feature of duplicate edges.
 
         If False, the simple graph will not have any edge features.
 
         (Default: False)
+    aggregator: str, optional
+        Indicate how to coalesce edge feature of duplicate edges.
+        If ``arbitrary``, select one of the duplicate edges' feature.
+        If ``sum``, compute the summation of duplicate edges' feature.
+        If ``mean``, compute the average of duplicate edges' feature.
+
+        (Default: ``arbitrary``)
 
     Returns
     -------
@@ -1991,14 +2051,8 @@ def to_simple(g,
         node_frames = utils.extract_node_subframes(g, None)
         utils.set_new_frames(simple_graph, node_frames=node_frames)
     if copy_edata:
-        eids = []
-        for i in range(len(g.canonical_etypes)):
-            feat_idx = F.asnumpy(edge_maps[i])
-            _, indices = np.unique(feat_idx, return_index=True)
-            eids.append(F.zerocopy_from_numpy(indices))
-
-        edge_frames = utils.extract_edge_subframes(g, eids)
-        utils.set_new_frames(simple_graph, edge_frames=edge_frames)
+        new_edge_frames = _coalesce_edge_frame(g, edge_maps, counts, aggregator)
+        utils.set_new_frames(simple_graph, edge_frames=new_edge_frames)
 
     if return_counts is not None:
         for count, canonical_etype in zip(counts, g.canonical_etypes):

src/array/cpu/segment_reduce.cc

@@ -40,6 +40,16 @@ void SegmentReduce(
   }
 }
 
+/*! \brief Scatter Add.*/
+template <int XPU, typename IdType, int bits>
+void ScatterAdd(NDArray feat,
+                NDArray idx,
+                NDArray out) {
+  SWITCH_BITS(bits, DType, {
+    cpu::ScatterAdd<IdType, DType>(feat, idx, out);
+  });
+}
+
 /*! \brief Backward function of segment cmp.*/
 template <int XPU, typename IdType, int bits>
 void BackwardSegmentCmp(
@@ -87,6 +97,30 @@ template void SegmentReduce<kDLCPU, int64_t, 64>(
     NDArray offsets,
     NDArray out,
     NDArray arg);
+template void ScatterAdd<kDLCPU, int32_t, 16>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLCPU, int64_t, 16>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLCPU, int32_t, 32>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLCPU, int64_t, 32>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLCPU, int32_t, 64>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLCPU, int64_t, 64>(
+    NDArray feat,
+    NDArray arg,
+    NDArray out);
 template void BackwardSegmentCmp<kDLCPU, int32_t, 16>(
     NDArray feat,
     NDArray arg,

src/array/cpu/segment_reduce.h

@@ -72,8 +72,35 @@ void SegmentCmp(NDArray feat, NDArray offsets,
   }
 }
 
+/*!
+ * \brief CPU kernel of Scatter Add (on first dimension) operator.
+ * \note math equation: out[idx[i], *] += feat[i, *]
+ * \param feat The input tensor.
+ * \param idx The indices tensor.
+ * \param out The output tensor.
+ */
+template <typename IdType, typename DType>
+void ScatterAdd(NDArray feat, NDArray idx, NDArray out) {
+  int n = feat->shape[0];
+  int dim = 1;
+  for (int i = 1; i < out->ndim; ++i)
+    dim *= out->shape[i];
+  const DType* feat_data = feat.Ptr<DType>();
+  const IdType* idx_data = idx.Ptr<IdType>();
+  DType* out_data = out.Ptr<DType>();
+#pragma omp parallel for
+  for (int i = 0; i < n; ++i) {
+    const int write_row = idx_data[i];
+    for (int k = 0; k < dim; ++k) {
+#pragma omp atomic
+      out_data[write_row * dim + k] += feat_data[i * dim + k];
+    }
+  }
+}
+
 /*!
  * \brief CPU kernel of backward phase of segment min/max.
+ * \note math equation: out[arg[i, k], k] = feat[i, k]
  * \param feat The input tensor.
  * \param arg The argmin/argmax tensor.
  * \param out The output tensor.

src/array/cuda/segment_reduce.cu

@@ -38,6 +38,16 @@ void SegmentReduce(const std::string& op,
 }
 
 
+template <int XPU, typename IdType, int bits>
+void ScatterAdd(NDArray feat,
+                NDArray idx,
+                NDArray out) {
+  SWITCH_BITS(bits, DType, {
+    cuda::ScatterAdd<IdType, DType>(feat, idx, out);
+  });
+}
+
+
 template <int XPU, typename IdType, int bits>
 void BackwardSegmentCmp(NDArray feat,
                         NDArray arg,
@@ -84,6 +94,30 @@ template void SegmentReduce<kDLGPU, int64_t, 64>(
     NDArray offsets,
     NDArray out,
     NDArray arg);
+template void ScatterAdd<kDLGPU, int32_t, 16>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLGPU, int64_t, 16>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLGPU, int32_t, 32>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLGPU, int64_t, 32>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLGPU, int32_t, 64>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
+template void ScatterAdd<kDLGPU, int64_t, 64>(
+    NDArray feat,
+    NDArray idx,
+    NDArray out);
 template void BackwardSegmentCmp<kDLGPU, int32_t, 16>(
     NDArray feat,
     NDArray arg,

src/array/cuda/segment_reduce.cuh

@@ -8,6 +8,7 @@
 
 #include "../../runtime/cuda/cuda_common.h"
 #include "./utils.h"
+#include "./atomic.cuh"
 
 namespace dgl {
 
@@ -27,17 +28,38 @@ __global__ void SegmentReduceKernel(
     const DType* feat, const IdType* offsets,
     DType* out, IdType* arg,
     int64_t n, int64_t dim){
-  int row = blockIdx.x;
-  int col = blockIdx.y * blockDim.x + threadIdx.x;
-  if (col < dim) {
-    DType local_accum = ReduceOp::zero();
-    IdType local_arg = -1;
-    for (IdType i = offsets[row]; i < offsets[row + 1]; ++i) {
-      ReduceOp::Call(&local_accum, &local_arg, feat[i * dim + col], i);
+  for (int row = blockIdx.x; row < n; row += gridDim.x) {
+    int col = blockIdx.y * blockDim.x + threadIdx.x;
+    while (col < dim) {
+      DType local_accum = ReduceOp::zero();
+      IdType local_arg = -1;
+      for (IdType i = offsets[row]; i < offsets[row + 1]; ++i) {
+        ReduceOp::Call(&local_accum, &local_arg, feat[i * dim + col], i);
+      }
+      out[row * dim + col] = local_accum;
+      if (ReduceOp::require_arg)
+        arg[row * dim + col] = local_arg;
+      col += gridDim.y * blockDim.x;
+    }
+  }
+}
+
+/*!
+ * \brief CUDA kernel of scatter add.
+ * \note each blockthread is responsible for adding a row in feature tensor
+ *       to a target row in output tensor.
+ */
+template <typename IdType, typename DType>
+__global__ void ScatterAddKernel(
+    const DType *feat, const IdType *idx, DType *out,
+    int64_t n, int64_t dim) {
+  for (int row = blockIdx.x; row < n; row += gridDim.x) {
+    const int write_row = idx[row];
+    int col = blockIdx.y * blockDim.x + threadIdx.x;
+    while (col < dim) {
+      cuda::AtomicAdd(out + write_row * dim + col, feat[row * dim + col]);
+      col += gridDim.y * blockDim.x;
     }
-    out[row * dim + col] = local_accum;
-    if (ReduceOp::require_arg)
-      arg[row * dim + col] = local_arg;
   }
 }
 
@@ -50,12 +72,14 @@ template <typename IdType, typename DType>
 __global__ void BackwardSegmentCmpKernel(
     const DType *feat, const IdType *arg, DType *out,
     int64_t n, int64_t dim) {
-  int row = blockIdx.x;
-  int col = blockIdx.y * blockDim.x + threadIdx.x;
-  if (col < dim) {
-    int write_row = arg[row * dim + col];
-    if (write_row >= 0) {
-      out[write_row * dim + col] = feat[row * dim + col];
+  for (int row = blockIdx.x; row < n; row += gridDim.x) {
+    int col = blockIdx.y * blockDim.x + threadIdx.x;
+    while (col < dim) {
+      int write_row = arg[row * dim + col];
+      if (write_row >= 0) {
+        out[write_row * dim + col] = feat[row * dim + col];
+      }
+      col += gridDim.y * blockDim.x;
     }
   }
 }
@@ -84,9 +108,9 @@ void SegmentReduce(
   for (int i = 1; i < out->ndim; ++i)
     dim *= out->shape[i];
 
-  const int nbx = n;
+  const int nbx = FindNumBlocks<'x'>(n);
   const int ntx = FindNumThreads(dim);
-  const int nby = (dim + ntx - 1) / ntx;
+  const int nby = FindNumBlocks<'y'>((dim + ntx - 1) / ntx);
   const int nty = 1;
   const dim3 nblks(nbx, nby);
   const dim3 nthrs(ntx, nty);
@@ -97,8 +121,43 @@ void SegmentReduce(
       n, dim);
 }
 
+/*!
+ * \brief CUDA implementation of Scatter Add (on first dimension).
+ * \note math equation: out[idx[i], *] += feat[i, *]
+ * \param feat The input tensor.
+ * \param idx The indices tensor.
+ * \param out The output tensor.
+ */
+template <typename IdType, typename DType>
+void ScatterAdd(
+    NDArray feat,
+    NDArray idx,
+    NDArray out) {
+  const DType* feat_data = feat.Ptr<DType>();
+  const IdType* idx_data = idx.Ptr<IdType>();
+  DType *out_data = out.Ptr<DType>();
+  
+  auto *thr_entry = runtime::CUDAThreadEntry::ThreadLocal();
+  int64_t n = feat->shape[0];
+  int64_t dim = 1;
+  for (int i = 1; i < out->ndim; ++i)
+    dim *= out->shape[i];
+
+  const int nbx = FindNumBlocks<'x'>(n);
+  const int ntx = FindNumThreads(dim);
+  const int nby = FindNumBlocks<'y'>((dim + ntx - 1) / ntx);
+  const int nty = 1;
+  const dim3 nblks(nbx, nby);
+  const dim3 nthrs(ntx, nty);
+  CUDA_KERNEL_CALL((ScatterAddKernel<IdType, DType>),
+                   nblks, nthrs, 0, thr_entry->stream,
+                   feat_data, idx_data, out_data,
+                   n, dim);
+}
+
 /*!
  * \brief CUDA implementation of backward phase of Segment Reduce with Min/Max reducer.
+ * \note math equation: out[arg[i, k], k] = feat[i, k]
  * \param feat The input tensor.
  * \param arg The ArgMin/Max information, used for indexing.
  * \param out The output tensor.
@@ -118,9 +177,9 @@ void BackwardSegmentCmp(
   for (int i = 1; i < out->ndim; ++i)
     dim *= out->shape[i];
 
-  const int nbx = n;
+  const int nbx = FindNumBlocks<'x'>(n);
   const int ntx = FindNumThreads(dim);
-  const int nby = (dim + ntx - 1) / ntx;
+  const int nby = FindNumBlocks<'y'>((dim + ntx - 1) / ntx);
   const int nty = 1;
   const dim3 nblks(nbx, nby);
   const dim3 nthrs(ntx, nty);

src/array/kernel.cc

@@ -153,6 +153,17 @@ void SegmentReduceDispatch(const std::string& op,
   });
 }
 
+/*! \brief Scatter Add (on first dimension) dispatch function. */
+void ScatterAddDispatch(NDArray feat, NDArray idx, NDArray out) {
+  ATEN_XPU_SWITCH_CUDA(feat->ctx.device_type, XPU, "ScatterAdd", {
+    ATEN_ID_TYPE_SWITCH(idx->dtype, IdType, {
+      ATEN_FLOAT_BITS_SWITCH(feat->dtype, bits, "Feature data", {
+        ScatterAdd<XPU, IdType, bits>(feat, idx, out);
+      });
+    });
+  });
+}
+
 /*! \brief Backward segment cmp dispatch function.*/
 void BackwardSegmentCmpDispatch(NDArray feat, NDArray arg, NDArray out) {
   ATEN_XPU_SWITCH_CUDA(feat->ctx.device_type, XPU, "BackwardSegmentCmp", {
@@ -225,6 +236,16 @@ DGL_REGISTER_GLOBAL("sparse._CAPI_DGLKernelSegmentReduce")
     SegmentReduceDispatch(op, feat, offsets, out, arg);
   });
 
+DGL_REGISTER_GLOBAL("sparse._CAPI_DGLKernelScatterAdd")
+.set_body([](DGLArgs args, DGLRetValue *rv) {
+    NDArray feat = args[0];
+    NDArray idx = args[1];
+    NDArray out = args[2];
+    CheckCtx(feat->ctx, {feat, idx, out}, {"feat", "idx", "out"});
+    CheckContiguous({feat, idx, out}, {"feat", "idx", "out"});
+    ScatterAddDispatch(feat, idx, out);
+  });
+
 DGL_REGISTER_GLOBAL("sparse._CAPI_DGLKernelBwdSegmentCmp")
 .set_body([](DGLArgs args, DGLRetValue *rv) {
     NDArray feat = args[0];

src/array/kernel_decl.h

@@ -76,6 +76,14 @@ void SegmentReduce(const std::string& op,
                    NDArray out,
                    NDArray arg);
 
+/*!
+ * \brief Scatter Add on first dimension.
+ */
+template <int XPU, typename IdType, int bits>
+void ScatterAdd(NDArray feat,
+                NDArray idx,
+                NDArray out);
+
 /*!
  * \brief Backward function of segment cmp.
  */

tests/compute/test_transform.py

@@ -772,6 +772,14 @@ def test_to_simple(idtype):
     assert 'h' not in sg.ndata
     assert 'h' not in sg.edata
 
+    # test coalesce edge feature
+    sg = dgl.to_simple(g, copy_edata=True, aggregator='arbitrary')
+    assert F.allclose(sg.edata['h'][1], F.tensor([4.]))
+    sg = dgl.to_simple(g, copy_edata=True, aggregator='sum')
+    assert F.allclose(sg.edata['h'][1], F.tensor([10.]))
+    sg = dgl.to_simple(g, copy_edata=True, aggregator='mean')
+    assert F.allclose(sg.edata['h'][1], F.tensor([5.]))
+
     # heterogeneous graph
     g = dgl.heterograph({
         ('user', 'follow', 'user'): ([0, 1, 2, 1, 1, 1],