Make utils for generating `dgl` or `pyg` graphs from processed data

[RemyLau]

Some current graph transforms methods directly save the processed dgl graphs in .uns. The limitations for doing so are

• Limits the choice of the framework (dgl vs. pyg) to use in the downstream model.
• .uns is not a good place to store large data

Solution

• Create utils that generate dgl or pyg graphs given raw feature, edge data, or adjacency matrix.
• Pass the raw data to method's fit function and construct the graph using the appropriate framework within the fit function.