Solved by qnl in post #3 
              
              
                I solved it by myself. 
t0 = torch.FloatTensor([[0.0,1.0,2.0]]).to_sparse() # not "[0,1,2]" but "[[0,1,2]]"
t1 = torch.FloatTensor([[3.0,4.0,5.0]]).to_sparse()
t2 = torch.cat([t0,t1])    # [[0,1,2],[3,4,5]] 

I misunderstood “3” is “1x3”. 
Thanks