[GraphBolt] ItemSetDict should inherit from Dict/Mapping

## 🔨Work Item

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Project tracker: https://github.com/orgs/dmlc/projects/2

## Description
For now, `ItemSetDict` is like a dictionary but not a real one as dict-like operations such as `pop()`, `keys()` are not supported. But these methods are very helpful for beginner to check what data are stored in `ItemSetDict`.
One solution is make `ItemSetDict` inherit from `Mapping` which offers dict-like methods in nature. And for the special case: iterating over data instead of keys in GraphBolt's DataLoader, we could just offer a dedicated **iterator** for it. Then `ItemSetDict` supports iterating over keys(which is like dict) and iterating over data(nodes/edges dimensions for `ItemSampler` in GraphBolt) simultaneously.

## Depending work items or issues