The implementation for get_edge_num in ArrowFragment is costly, and incorrect

[sighingnow]

Describe your problem

There's a todo: https://github.com/v6d-io/v6d/blob/main/modules/graph/fragment/arrow_fragment.vineyard-mod#L690 and the logic to compute total edges for directed graphs is incorrect: https://github.com/v6d-io/v6d/blob/main/modules/graph/fragment/arrow_fragment.vineyard-mod#L288

Additional context

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