Add Evaluation Code for Scene Graph Generation

[JackCai1206]

I have added code to fintune and evaluate OFA on Scene Graph Generation task. I have used a popular evaluation codebase from the SGG literature to calculate the metrics.

Specifically, I take the data from visual genome, and linearize the graphs into a string. I finetune OFA using image as the input and the graph-string as the output.

Would the OFA team be interested in this new feature?