Encounter "Invalid vertex type: 1" error when training MoNet

## 🐛 Bug

When I try to train MoNet with builtin GMMConv module, I got errors as below:
`Traceback (most recent call last):
File "/home/Wuyinwei/Desktop/Distill_GCN_Experiment/test_monet.py", line 116, in
output = model(blocks, batch_inputs)
File "/home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1130, in _call_impl
return forward_call(*input, **kwargs)
File "/home/Wuyinwei/Desktop/Distill_GCN_Experiment/test_monet.py", line 56, in forward
h = self.layers[i](
File "/home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1130, in _call_impl
return forward_call(*input, **kwargs)
File "/home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/nn/pytorch/conv/gmmconv.py", line 220, in forward
if (graph.in_degrees() == 0).any():
File "/home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/heterograph.py", line 3433, in in_degrees
v = self.dstnodes(dsttype)
File "/home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/view.py", line 51, in __call__
self._graph._graph.number_of_nodes(ntid),
File "/home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/heterograph_index.py", line 376, in number_of_nodes
return _CAPI_DGLHeteroNumVertices(self, int(ntype))
File "dgl/_ffi/_cython/./function.pxi", line 295, in dgl._ffi._cy3.core.FunctionBase.__call__
File "dgl/_ffi/_cython/./function.pxi", line 227, in dgl._ffi._cy3.core.FuncCall
File "dgl/_ffi/_cython/./function.pxi", line 217, in dgl._ffi._cy3.core.FuncCall3
dgl._ffi.base.DGLError: [10:56:50] /opt/dgl/src/graph/./heterograph.h:67: Check failed: meta_graph_->HasVertex(vtype): Invalid vertex type: 1
Stack trace:
[bt] (0) /home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/libdgl.so(dmlc::LogMessageFatal::~LogMessageFatal()+0x4f) [0x7fd7dd3ab30f]
[bt] (1) /home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/libdgl.so(dgl::HeteroGraph::NumVertices(unsigned long) const+0xa2) [0x7fd7dd72e162]
[bt] (2) /home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/libdgl.so(+0x733aed) [0x7fd7dd737aed]
[bt] (3) /home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/libdgl.so(DGLFuncCall+0x48) [0x7fd7dd6bee58]
[bt] (4) /home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/_ffi/_cy3/core.cpython-38-x86_64-linux-gnu.so(+0x163fc) [0x7fd8f9e943fc]
[bt] (5) /home/Wuyinwei/anaconda3/envs/ldgl/lib/python3.8/site-packages/dgl/_ffi/_cy3/core.cpython-38-x86_64-linux-gnu.so(+0x1692b) [0x7fd8f9e9492b]
[bt] (6) /home/Wuyinwei/anaconda3/envs/ldgl/bin/python(_PyObject_MakeTpCall+0x3eb) [0x4d14db]
[bt] (7) /home/Wuyinwei/anaconda3/envs/ldgl/bin/python(_PyEval_EvalFrameDefault+0x4f48) [0x4cc578]
[bt] (8) /home/Wuyinwei/anaconda3/envs/ldgl/bin/python() [0x4e8af7]`

I think it is a problem related to `MultiLayerFullNeighborSampler` or `DataLoader` since I got no errors when training without neighbor sampling.
## To Reproduce

Steps to reproduce the behavior:

The code I ran is at:
[test_monet.py](https://github.com/WUyinwei-hah/test_monet/blob/main/test_monet_Flickr.py)

## Environment

- DGL Version:
dgl 1.0.2+cu116
dglgo 0.0.2
- Backend Library & Version:
torch 1.12.1+cu116
torch-geometric 2.1.0
- OS:
linux ubuntu 20
- How you installed DGL:
pip install dgl -f https://data.dgl.ai/wheels/cu116/repo.html
pip install dglgo -f https://data.dgl.ai/wheels-test/repo.html
- Python version:
3.8
- CUDA/cuDNN version (if applicable):
11.3
- GPU models and configuration:
RTX3090
- Any other relevant information:

## Additional context