I am not sure 100% what is singleton dimension of a tensor. I think this may be the dimension where we have 1: torch.rand(1, 2, 3, 4, 5). Can someone confirm what is this?
I found this in:

RuntimeError: The size of tensor a (5000) must match the size of tensor b (60) at non-singleton dimension 2

I am not sure what singleton means in PyTorch tensor.