Corner Case Handling for Self-Loops and Single-Node Graphs

[ak47mrj]

I encountered two corner cases that result in incorrect behavior when reading graphs with self-loops or single-node graphs. These issues may affect downstream tasks that rely on accurate graph structure and attributes.

1. Self-Loop Weight Doubling: When reading a graph with a self-loop (an edge where src == dst), the weight of the self-loop is incorrectly doubled. For example, if the original edge weight is 1, the weight returns it as 2.
2. Incorrect n_edges Attribute for Single-Node Graph with Self-Loop: When reading a graph that contains only a single node with a self-loop, the n_edges attribute is incorrectly set to 0.