[GraphBolt] ItemSetDict should inherit from Dict/Mapping

[Rhett-Ying]

🔨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