[mlir python] Add nanobind support for standalone dialects. (#117922)

This PR allows out-of-tree dialects to write Python dialect modules
using nanobind instead of pybind11.

It may make sense to migrate in-tree dialects and some of the ODS Python
infrastructure to nanobind, but that is a topic for a future change.

This PR makes the following changes:
* adds nanobind to the CMake and Bazel build systems. We also add
robin_map to the Bazel build, which is a dependency of nanobind.
* adds a PYTHON_BINDING_LIBRARY option to various CMake functions, such
as declare_mlir_python_extension, allowing users to select a Python
binding library.
* creates a fork of mlir/include/mlir/Bindings/Python/PybindAdaptors.h
named NanobindAdaptors.h. This plays the same role, using nanobind
instead of pybind11.
* splits CollectDiagnosticsToStringScope out of PybindAdaptors.h and
into a new header mlir/include/mlir/Bindings/Python/Diagnostics.h, since
it is code that is no way related to pybind11 or for that matter,
Python.
* changed the standalone Python extension example to have both pybind11
and nanobind variants.
* changed mlir/python/mlir/dialects/python_test.py to have both pybind11
and nanobind variants.

Notes:
* A slightly unfortunate thing that I needed to do in the CMake
integration was to use FindPython in addition to FindPython3, since
nanobind's CMake integration expects the Python_ names for variables.
Perhaps there's a better way to do this.

Author: hawkinsp

mlir/include/mlir/Bindings/Python/Diagnostics.h

@@ -0,0 +1,59 @@
+//===- Diagnostics.h - Helpers for diagnostics in Python bindings ---------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_BINDINGS_PYTHON_DIAGNOSTICS_H
+#define MLIR_BINDINGS_PYTHON_DIAGNOSTICS_H
+
+#include <cassert>
+#include <string>
+
+#include "mlir-c/Diagnostics.h"
+#include "mlir-c/IR.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace mlir {
+namespace python {
+
+/// RAII scope intercepting all diagnostics into a string. The message must be
+/// checked before this goes out of scope.
+class CollectDiagnosticsToStringScope {
+public:
+  explicit CollectDiagnosticsToStringScope(MlirContext ctx) : context(ctx) {
+    handlerID = mlirContextAttachDiagnosticHandler(ctx, &handler, &errorMessage,
+                                                   /*deleteUserData=*/nullptr);
+  }
+  ~CollectDiagnosticsToStringScope() {
+    assert(errorMessage.empty() && "unchecked error message");
+    mlirContextDetachDiagnosticHandler(context, handlerID);
+  }
+
+  [[nodiscard]] std::string takeMessage() { return std::move(errorMessage); }
+
+private:
+  static MlirLogicalResult handler(MlirDiagnostic diag, void *data) {
+    auto printer = +[](MlirStringRef message, void *data) {
+      *static_cast<std::string *>(data) +=
+          llvm::StringRef(message.data, message.length);
+    };
+    MlirLocation loc = mlirDiagnosticGetLocation(diag);
+    *static_cast<std::string *>(data) += "at ";
+    mlirLocationPrint(loc, printer, data);
+    *static_cast<std::string *>(data) += ": ";
+    mlirDiagnosticPrint(diag, printer, data);
+    return mlirLogicalResultSuccess();
+  }
+
+  MlirContext context;
+  MlirDiagnosticHandlerID handlerID;
+  std::string errorMessage = "";
+};
+
+} // namespace python
+} // namespace mlir
+
+#endif // MLIR_BINDINGS_PYTHON_DIAGNOSTICS_H