Improve diagnostic handler

It wasn't printing notes or putting the "error:" in front.

frontends/pytorch/csrc/builder/module_builder.cpp

@@ -55,23 +55,47 @@ static MlirModule createEmptyModule(MlirContext context) {
   return mlirModuleCreateEmpty(loc);
 }
 
+static std::string
+stringifyMlirDiagnosticSeverity(MlirDiagnosticSeverity severity) {
+  switch (severity) {
+  case MlirDiagnosticError:
+    return "error";
+  case MlirDiagnosticWarning:
+    return "warning";
+  case MlirDiagnosticNote:
+    return "note";
+  case MlirDiagnosticRemark:
+    return "remark";
+  default:
+    return "<unknown severity>";
+  }
+}
+
+static void printDiagnostic(MlirDiagnostic diagnostic) {
+  std::stringstream ss;
+  ss << stringifyMlirDiagnosticSeverity(mlirDiagnosticGetSeverity(diagnostic))
+     << ": ";
+  auto stringCallback = [](MlirStringRef s, void *stringCallbackUserData) {
+    auto *ssp = static_cast<std::stringstream *>(stringCallbackUserData);
+    ssp->write(s.data, s.length);
+  };
+  mlirDiagnosticPrint(diagnostic, stringCallback, static_cast<void *>(&ss));
+  // Use pybind11's print:
+  // https://pybind11.readthedocs.io/en/stable/advanced/pycpp/utilities.html
+  py::print(ss.str(),
+            py::arg("file") = py::module_::import("sys").attr("stderr"));
+}
+
 // Register a diagnostic handler that will redirect output to `sys.stderr`
 // instead of a C/C++-level file abstraction. This ensures, for example,
 // that mlir diagnostics emitted are correctly routed in Jupyter notebooks.
 static void registerPythonSysStderrDiagnosticHandler(MlirContext context) {
   auto diagnosticHandler = [](MlirDiagnostic diagnostic,
                               void *) -> MlirLogicalResult {
-    std::stringstream ss;
-    auto stringCallback =
-        [](MlirStringRef s, void *stringCallbackUserData) {
-      auto *ssp = static_cast<std::stringstream *>(stringCallbackUserData);
-      ssp->write(s.data, s.length);
-    };
-    mlirDiagnosticPrint(diagnostic, stringCallback, static_cast<void *>(&ss));
-    // Use pybind11's print:
-    // https://pybind11.readthedocs.io/en/stable/advanced/pycpp/utilities.html
-    using namespace pybind11::literals;
-    py::print(ss.str(), "file"_a = py::module_::import("sys").attr("stderr"));
+    printDiagnostic(diagnostic);
+    for (int i = 0, e = mlirDiagnosticGetNumNotes(diagnostic); i != e; i++) {
+      printDiagnostic(mlirDiagnosticGetNote(diagnostic, i));
+    }
     return mlirLogicalResultSuccess();
   };
   MlirDiagnosticHandlerID id = mlirContextAttachDiagnosticHandler(