ggml : add note about mul_mat_id + disable mul_mat_id_dup tests (#30501)

ggml/include/ggml.h

@@ -1497,6 +1497,11 @@ extern "C" {
             enum ggml_op_hint    hint);
 
     // indirect matrix multiplication
+    //
+    // [TAG_MUL_MAT_ID_DUPLICATE_ID]
+    // note: this op has undefined behaviour if a given `a` is referenced more than once for a given `b`
+    //       (i.e. duplicate experts per token are not guaranteed to be supported in all backends)
+    // TODO: add backend support: https://github.com/ggml-org/llama.cpp/pull/29998#issuecomment-6153742533
     GGML_API struct ggml_tensor * ggml_mul_mat_id(
             struct ggml_context * ctx,
             struct ggml_tensor  * as,

tests/test-backend-ops.cpp

@@ -11028,6 +11028,8 @@ static std::vector<std::unique_ptr<test_case>> make_test_cases_eval() {
     test_cases.emplace_back(new test_mul_mat_id(GGML_TYPE_TQ1_0, GGML_TYPE_F32, 28, 10, false, 1024, 1, 4096));
     test_cases.emplace_back(new test_mul_mat_id(GGML_TYPE_TQ1_0, GGML_TYPE_F32, 128, 8, false, 1024, 1, 2048));
 
+    // TODO: disabled until CUDA backend support is added [TAG_MUL_MAT_ID_DUPLICATE_ID]
+#if 0
     // repeated expert ids within a row
     for (ggml_type ta : {GGML_TYPE_F16, GGML_TYPE_Q8_0, GGML_TYPE_Q4_0}) {
         for (int n : {9, 16, 33, 64}) {
@@ -11039,6 +11041,7 @@ static std::vector<std::unique_ptr<test_case>> make_test_cases_eval() {
         test_cases.emplace_back(new test_mul_mat_id_dup(ta, GGML_TYPE_F32, 1025, 10, false, 64, 33, 256, 1.0f, 0, true));
         test_cases.emplace_back(new test_mul_mat_id_dup(ta, GGML_TYPE_F32, 2050, 10, true, 64, 33, 256));
     }
+#endif
 
     for (ggml_type type_a : all_types) {
         test_cases.emplace_back(new test_mul_mat_id(type_a, GGML_TYPE_F32, 4, 2, false, 64, 16, 3*ggml_blck_size(type_a)));