Cloning DGLBlock breaks underyling metagraph, making it unusable.

## 🐛 Bug

When a DGLBlock is cloned, the duplicate srctype and dsttype get re-merged in the metagraph, which makes it unusable. This appears to be the result of the metagraph being constructed purely from the meta-edges, losing information about src and dst types being distinct: https://github.com/dmlc/dgl/blob/master/python/dgl/heterograph.py#L5604

## To Reproduce

On the output of a block from the dataloader call .clone() and attempt to print the graph:
```
Traceback (most recent call last):
File "node_classification.py", line 143, in
train(args, device, g, dataset, model)
File "node_classification.py", line 106, in train
print(blocks[0].clone())
File "/home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/heterograph.py", line 6383, in __repr__
dstnode=self.number_of_dst_nodes(),
File "/home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/heterograph.py", line 2415, in number_of_dst_nodes
return self.num_dst_nodes(ntype)
File "/home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/heterograph.py", line 2475, in num_dst_nodes
for nty in self.dsttypes])
File "/home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/heterograph.py", line 2475, in
for nty in self.dsttypes])
File "/home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/heterograph_index.py", line 337, in number_of_nodes
return _CAPI_DGLHeteroNumVertices(self, int(ntype))
File "dgl/_ffi/_cython/./function.pxi", line 293, in core.FunctionBase.__call__
File "dgl/_ffi/_cython/./function.pxi", line 225, in core.FuncCall
File "dgl/_ffi/_cython/./function.pxi", line 215, in core.FuncCall3
dgl._ffi.base.DGLError: [17:49:41] /home/dominique/src/dgl/src/graph/./heterograph.h:76: Check failed: meta_graph_->HasVertex(vtype): Invalid vertex type: 1
Stack trace:
[bt] (0) /home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/libdgl.so(+0x6e0332) [0x7f232ea13332]
[bt] (1) /home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/libdgl.so(dgl::HeteroGraph::NumVertices(unsigned long) const+0x9d) [0x7f232ea1bc5d]
[bt] (2) /home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/libdgl.so(+0x6f127e) [0x7f232ea2427e]
[bt] (3) /home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/libdgl.so(DGLFuncCall+0x73) [0x7f232e9a7c93]
[bt] (4) /home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/_ffi/_cy3/core.cpython-36m-x86_64-linux-gnu.so(+0x19a03) [0x7f22fc49ca03]
[bt] (5) /home/dominique/.local/lib/python3.6/site-packages/dgl-0.9-py3.6-linux-x86_64.egg/dgl/_ffi/_cy3/core.cpython-36m-x86_64-linux-gnu.so(+0x1a7a7) [0x7f22fc49d7a7]
[bt] (6) python3(_PyObject_FastCallKeywords+0x19c) [0x5a8b3c]
[bt] (7) python3() [0x50a9a3]
[bt] (8) python3(_PyEval_EvalFrameDefault+0x444) [0x50c414]
```