Detail about 'background' class

[develop-productivity]

Hi, thanks for your great work.

I have a question about the classifier in this paper, I want to know whether you used the 'background' class in the $C_{train}$.

We encode the names of all the categories in Ctrain with the frozen text encoder and define the set of embeddings of all the training categories' names as: Equation4

If you used a background class, is it learnable or fixed?