I think this  N-pair implementation is easier to use.
embeddings_anchor => code_embedding,
embeddings_positive => docstring_embedding,
labels => [1,2,3,......N],
Where N is the batch_size.
I usually use N-pair loss in a different context where there is a set of pre-defined classes, each class has a fixed label. This is probably invalid for your problem setting.
Thus, I think setting labels = [1,2,3,......N] would work. It will signals that each code_embedding is different. I doubt the exact values matter. They just need to be different values.