Print backend name on test-backend-ops failure (#4751)

tests/test-backend-ops.cpp

@@ -392,15 +392,21 @@ struct test_case {
         struct callback_userdata {
             bool   ok;
             double max_err;
+            ggml_backend_t backend1;
+            ggml_backend_t backend2;
         };
 
         callback_userdata ud {
             true,
             max_nmse_err(),
+            backend1,
+            backend2
         };
 
         auto callback = [](int index, ggml_tensor * t1, ggml_tensor * t2, void * user_data) -> bool {
             callback_userdata * ud = (callback_userdata *) user_data;
+            const char * bn1 = ggml_backend_name(ud->backend1);
+            const char * bn2 = ggml_backend_name(ud->backend2);
 
             if (t1->op == GGML_OP_NONE) {
                 // sentinels must be unchanged
@@ -422,7 +428,7 @@ struct test_case {
             for (size_t i = 0; i < f1.size(); i++) {
                 // check for nans
                 if (std::isnan(f1[i]) || std::isnan(f2[i])) {
-                    printf("[%s] NaN at index %zu (%f %f) ", ggml_op_desc(t1), i, f1[i], f2[i]);
+                    printf("[%s] NaN at index %zu (%s=%f %s=%f) ", ggml_op_desc(t1), i, bn1, f1[i], bn2, f2[i]);
                     ud->ok = false;
                     return true;
                 }
@@ -430,12 +436,12 @@ struct test_case {
                 if (isinf_or_max(f1[i]) || isinf_or_max(f2[i])) {
                     if (isinf_or_max(f1[i]) && isinf_or_max(f2[i])) {
                         if (std::signbit(f1[i]) != std::signbit(f2[i])) {
-                            printf("[%s] inf sign mismatch: %f %f ", ggml_op_desc(t1), f1[i], f2[i]);
+                            printf("[%s] inf sign mismatch: %s=%f %s=%f ", ggml_op_desc(t1), bn1, f1[i], bn2, f2[i]);
                             ud->ok = false;
                             return true;
                         }
                     } else {
-                        printf("[%s] inf mismatch: %f %f ", ggml_op_desc(t1), f1[i], f2[i]);
+                        printf("[%s] inf mismatch: %s=%f %s=%f ", ggml_op_desc(t1), bn1, f1[i], bn2, f2[i]);
                         ud->ok = false;
                         return true;
                     }