CombinedReducers MDRange support

[dhollman]

Implement feature request in #3394

[dhollman]

@dalg24 I have no idea why clang-format changed some other things in the file that I didn't touch. Any thoughts? Never mind; I figured it out.

[dalg24]

Why aren't you taking advantage of FTAD?

[dalg24]

Why aren't you taking advantage of FTAD?

I pulled your changes and I see that my suggestion compiles but produces a bug. Please educate me.

[crtrott]

OK I don't understand it either :-) @nliber @dhollman so if you could educate us what is going on maybe with a smaller example that would be awesome!

[masterleinad]

/Users/travis/build/kokkos/kokkos/core/unit_test/TestReduce.hpp:523:63: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
      Kokkos::MDRangePolicy<TEST_EXECSPACE, Kokkos::Rank<3>>({0, 0, 0},
                                                              ^~~~~~~
                                                              {      }
/Users/travis/build/kokkos/kokkos/core/unit_test/TestReduce.hpp:524:63: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
                                                             {nw, 1, 1}),
                                                              ^~~~~~~~
                                                              {       }

[dhollman]

@dalg24 @crtrott There's actually nothing really complicated going on here. You can't use FTAD here because you can't deduce a variadic parameter pack that's not at the end of the parameter list, so you have to give the first parameter pack explicitly at the call site.

[crtrott]

Ah ok I get it now.