Implement the function unique that takes as a parameter a two-dimensional list and returns a one-dimensional list containing all the unique entries in the list. The entries in the list returned do not need to be ordered in any particular way. For example, in the list [[1, 0, 1], [0, 1, 0]] there are only two unique values (0 and 1) so the function would return the list [0, 1] (or the list [1, 0] -- both are valid). The list passed as a parameter should not be altered by the function.
>>> unique([[1, 5, 2, 1], [1, 4, 3, 5], [], [5]])
[1, 5, 2, 4, 3]