Thanks for your explanation, but this loss will be used in unsupervised learning or semi supervised learning.
so c is not the target.
based on the documentation link, the c is a Tensor label  1 or -1. This is used for measuring whether two inputs are similar or dissimilar; my understanding was that it should be in the similar shape with a and b. though, if i change its size to be
c = torch.ones(1,1,10,10)

the second case witll work apparently.
But i still did not understand what is happening there.