Keyword Arguments:
      out (Tensor, optional) -- the output tensor.
Example:
  >>> a = torch.randn(3, 3)
  >>> a
  tensor([[-1.0813, -0.8619,  0.7105],
          [ 0.0935,  0.1380,  2.2112],
          [-0.3409, -0.9828,  0.0289]])
  >>> torch.tril(a)
  tensor([[-1.0813,  0.0000,  0.0000],
          [ 0.0935,  0.1380,  0.0000],
          [-0.3409, -0.9828,  0.0289]])

  >>> b = torch.randn(4, 6)
  >>> b
  tensor([[ 1.2219,  0.5653, -0.2521, -0.2345,  1.2544,  0.3461],
          [ 0.4785, -0.4477,  0.6049,  0.6368,  0.8775,  0.7145],
          [ 1.1502,  3.2716, -1.1243, -0.5413,  0.3615,  0.6864],
          [-0.0614, -0.7344, -1.3164, -0.7648, -1.4024,  0.0978]])
  >>> torch.tril(b, diagonal=1)
  tensor([[ 1.2219,  0.5653,  0.0000,  0.0000,  0.0000,  0.0000],
          [ 0.4785, -0.4477,  0.6049,  0.0000,  0.0000,  0.0000],
          [ 1.1502,  3.2716, -1.1243, -0.5413,  0.0000,  0.0000],
