[refactor] Decouple model components from graph

[zhjwy9343]

Current issue:
In order to reuse GraphStorm's model components, e.g., InpuLayers and Encoders, users need to create a g(DistGraph) first. This create some barrier for users to build customized models based some built-in compoents.

Suggested solution:
Current GraphStorm components, in effect, only use g(DistGraph) to extract meta information of the graph, such as canonical_etypes or node feature dimensions. Therefore, it would be convenience to just pass those meta information as arguments, which will decouple the binding between model components and g(DistGraph)s.