Add bipartite graph support to `structured_negative_sampling`

🚀 The feature, motivation and pitch

Hello, First and foremost, I apologize for potentially misplacing this issue in the wrong section. When working with the negative sampling part and the necessity to use the function "structured_negative_sampling" there might be inaccuracies when dealing with a bipartite graph. This issue becomes more evident if the number of nodes on the right side of the bipartite graph exceeds the number of nodes on the left side. For instance, consider the movie dataset, which consists of 6000 users and 4000 items, often employed in link prediction and recommender systems. I'm uncertain whether adding this feature falls within the scope of this library. However, I bring it up because, in the contribution section, you mentioned that we should address any uncertainties with you.

Alternatives

No response

Additional context

No response