[MLIR][Python][Transform] Print diagnostics also upon success (#172188)

If we do not collect the diagnostics from the
CollectDiagnosticsToStringScope, even when the named_sequence applied
successfully, the Scope object's destructor will assert (with a
unhelpful message).

mlir/lib/Bindings/Python/TransformInterpreter.cpp

@@ -70,8 +70,17 @@ static void populateTransformInterpreterSubmodule(nb::module_ &m) {
 
         MlirLogicalResult result = mlirTransformApplyNamedSequence(
             payloadRoot, transformRoot, transformModule, options.options);
-        if (mlirLogicalResultIsSuccess(result))
+        if (mlirLogicalResultIsSuccess(result)) {
+          // Even in cases of success, we might have diagnostics to report:
+          std::string msg;
+          if ((msg = scope.takeMessage()).size() > 0) {
+            fprintf(stderr,
+                    "Diagnostic generated while applying "
+                    "transform.named_sequence:\n%s",
+                    msg.data());
+          }
           return;
+        }
 
         throw nb::value_error(
             ("Failed to apply named transform sequence.\nDiagnostic message " +