slice_batch tensor index

## 🚀 Feature

Now we can use slice_batch to get a sample from a batched graph.
Get the 1-st graph:
```
g = dgl.slice_batch(bg, 1)
```

A more advanced idex slicing will be useful:
Get the 1,2,5-th graph and concate then as a new batched graph
```
g = dgl.slice_batch(bg, [1,2,5])
```

A possible work around is to do unbatch and batch again:

```
g = dgl.batch(dgl.unbatch(bg)[:5])
```
but I do think slicing should be a fundamental function in dgl library