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# note that a is in dtype float32 (by default)
# the one to be cat has to be also 
# with dtype of float32.
a = torch.tensor(())
for i in range(3):
    # if i = torch.tensor(1)
    # it cannot be cat, since it has 
    # zero dimention.
    # Also use .float() to make sure that they 
    # are in the same dtype
    i = torch.tensor([i]).float()
    a = torch.cat((a, i), 0)
print(a)
tensor([1., 2., 3.])