[Refactor] Further decouple DLPack from DGL NDArray

## 🔨Work Item

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

## Description

The issue comes up from PR #4454 . The PR introduced a new data structure called `DGLArray` to replace `DLTensor` from DLPack. The follow up action is to further simplify the codebase. Considerations:
* Currrently, there are three classes `dgl::runtime::NDArray`, `dgl::runtime::NDArray::Container` and `dgl::DGLArray`. The question is whether we can consolidate them into one or two classes.
* Exposing DGL NDArray to Python adds another layer of complexity to the system. For example, there is a `dgl/_ffi/runtime_ctypes.py::DGLArray` python class. We need to further decouple it from DLPack and document clearly what each class is about.

@yaox12 let's use this thread for discussion.

## Depending work items or issues

None