Native support for batched graph saving/loading

## 🚀 Feature

support saving/loading batched graphs (by preserving the information for unbatching during saving).

## Motivation

It would greatly imporve the time efficiency (and maybe space too?) comparing with saving/loading `List[DGLGraph]`

## Alternatives

An alternative approach could involve storing additional metadata such as `batch_num_nodes` and `batch_num_edges` while saving batched graphs, which could then be used to reconstruct the batch structure after loading the batched graph. It seems to work on homogeneous graphs, not sure about the heterogeneous ones.

## Additional context

see https://github.com/dmlc/dgl/issues/936