[Feature] Random Walk with Restart terminates the random walk rather than restarting

## ❓ Questions and Help about Random Walk with Restart

Thanks to the developers for their efforts to provide us with an excellent framework.

The restart random walk module in version 0.4.3, `dgl.contrib.sampling.random_walk_with_restart`, is not enough to be replaced by `dgl.sampling.rand_walk` in the current version.

In the current version, the parameter `restart_prob` means that the walk is **"terminated"** with a certain probability instead of **"restarted"**.

As the following example shows, the walk is terminated without restarting.

```
import dgl
import networkx as nx

nxgraph = nx.Graph()
nxgraph.add_edges_from([(0,1), (1,2), (1,3), (2,0), (3,0)])
g1 = dgl.from_networkx(nxgraph)

print(dgl.sampling.random_walk(g1, [0], restart_prob=0.5, length=10))
```
output:
`>>> (tensor([[ 0, 2, 0, -1, -1, -1, -1, -1, -1, -1, -1]]), tensor([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]))`

This walk is shutting down at 0->2->0. So, what can I do for attaining "**restart**"?