Thank you for the suggestion. sym_toeplitz is a resembles want, but it is not quite the same thing.
from gpytorch import utils
c = torch.tensor([1, 6, 4, 5], dtype=torch.float)
res = utils.toeplitz.sym_toeplitz(c)
res
# tensor([[1., 6., 4., 5.],
#        [6., 1., 6., 4.],
#        [4., 6., 1., 6.],
#        [5., 4., 6., 1.]])