`node_type_subgraph` ignore batch structure of input graph

## 🐛 Bug

## To Reproduce
```python
g = dgl.batch([g0,g1,g2])
g.node_type_subgraph(['type1'])
print(g.batch_size)
```

## Expected behavior
I assumed the `node_type_subgraph` returns the subgraphs of inputs, hence the output `g` has to have batch size of 3.
But it actually give me a single graph.