[Bug] Fix moe_sum signature (#18440)

Signed-off-by: Bill Nell <bnell@redhat.com>

csrc/moe/torch_bindings.cpp

@@ -10,7 +10,7 @@ TORCH_LIBRARY_EXPAND(TORCH_EXTENSION_NAME, m) {
 
   // Calculate the result of moe by summing up the partial results
   // from all selected experts.
-  m.def("moe_sum(Tensor! input, Tensor output) -> ()");
+  m.def("moe_sum(Tensor input, Tensor! output) -> ()");
   m.impl("moe_sum", torch::kCUDA, &moe_sum);
 
   // Aligning the number of tokens to be processed by each expert such

tests/kernels/moe/test_moe.py

@@ -575,3 +575,21 @@ def test_moe_align_block_size_opcheck():
     opcheck(torch.ops._moe_C.moe_align_block_size,
             (topk_ids, num_experts, block_size, sorted_ids, expert_ids,
              num_tokens_post_pad))
+
+
+@pytest.mark.parametrize("m", [1, 33, 222, 1024 * 128])
+@pytest.mark.parametrize("topk", TOP_KS)
+@pytest.mark.parametrize("k", [128, 511, 1024])
+@pytest.mark.parametrize("dtype",
+                         [torch.float32, torch.float16, torch.bfloat16])
+@pytest.mark.skipif(current_platform.is_rocm(), reason="Skip for rocm")
+def test_moe_sum(m: int, topk: int, k: int, dtype: torch.dtype):
+    input = torch.randn((m, topk, k), device="cuda", dtype=dtype)
+    actual = torch.empty((m, k), device="cuda", dtype=dtype)
+
+    expected = input.sum(dim=1)
+    torch.ops._moe_C.moe_sum(input, actual)
+
+    torch.testing.assert_close(actual, expected, atol=2e-2, rtol=0)
+
+    opcheck(torch.ops._moe_C.moe_sum, (input, actual))