[RFC] Add a MFG back to Subgraph function

## 🚀 Feature
A utility function to revert a sampled MFGs to a DGLGraph in heterogeneous graphs.

Input: an MFGs that come from NeighborSampler and NodeDataLoader
Output: 3 objects
> A dictionary of node ids, where a key is a node types, and a value is a tuple of new subgraph node ids and original graph node ids.
> A dictionary of edges in the form of a tuple of a source node id list and a destination node list where ids are the **new subgraph node ids**.
> A dictionary of edges in the form of a tuple of a source node id list and a destination node list where ids are the **original graph node ids**.

## Motivation
To have various GNN model explainers work, it is required to extract the same subgraph that computes the final prediction for either visualization or explainer training. In mini-batch mode, DGL uses MFGs to facilitate computation. But MFGs cannot directly be used for visualization or explainer training. This is even true in heterogeneous graphs.

## Alternatives
No

## Pitch
No

## Additional context
I have written a function to meet the above requirements. But not PR yet.