I change the code from dense.mm(sparse) into sparse.mm(dense) as you told me. Then I encounter the same problem in that issue 1 with the error
RuntimeError: Expected object of type Variable[torch.cuda.sparse.FloatTensor] but found type Variable[torch.cuda.FloatTensor] for argument #1 'mat2'

sad~
The trouble is torch.mm(sparse, dense) works with tensors but not variables to sparse ops with autograd