Thank you for your reply.
“closed graph” means that model “a” parameters will update based on 15 samples, model ‘b’ parameters update on the next 20 samples and model ‘c’ parameters update on the last 25 samples respectively.   Although, the final loss is summations of all 3 models losses and loss gradient backward to update the individual model parameters. that means model ‘b’ would not depend on the first 15 samples and the last 25 samples respectively (because model b parameters don’t have closed connection between two sample spaces). This is my understanding. Maybe I am wrong.