[Unity] Add multinomial from uniform sample

[yongwww]

This PR introduces the multinomial_from_uniform into nn module for GPU case, and vm.builtin.multinomial_from_uniform for CPU. The multinomial_from_uniform returns a tensor wherein each row contains an index sampled from a multinomial probability distribution.

The test case is for testing the build and run, it has been commented out to avoid overloading the nn module op tests.

cc: @tqchen @vinx13 @MasterJH5574