
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.
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