Oh, I get it. A tuple indexes an element, a list provides multiple indexes in the first dimension of t.
In [31]: t = torch.rand(3,5) 
    ...: print(t[1,2])
    ...: print(t[1,2].data_ptr()) 
tensor(0.1442)                                                                                                                                                                                                          
140485274866972

In [32]: idx = (1,2) 
    ...: print(t[idx]) 
    ...: print(t[idx].data_ptr()) 
tensor(0.1442)
140485274866972
In [33]: idx = [1,2] 
    ...: print(t[idx])   
    ...: print(t[idx].data_ptr()) 
tensor([[0.8098, 0.4710, 0.1442, 0.5391, 0.1699],
        [0.7292, 0.6585, 0.7074, 0.4800, 0.6104]])
140485233816192