Should be able to reduce over size {0} dimensions.

[gchanan]

This currently fails, but should pass:

      Type & T = CPU(kFloat);
      auto t = T.ones({0});
      // FIXME: you should be able to reduce over size {0}
      try {
        t.sum(0);
        assert(false);
      } catch (std::runtime_error &e) {}
    }