[MLIR][python bindings] Add TypeCaster for returning refined types from python APIs

depends on D150839

This diff uses `MlirTypeID` to register `TypeCaster`s (i.e., `[](PyType pyType) -> DerivedTy { return pyType; }`) for all concrete types (i.e., `PyConcrete<...>`) that are then queried for (by `MlirTypeID`) and called in `struct type_caster<MlirType>::cast`. The result is that anywhere an `MlirType mlirType` is returned from a python binding, that `mlirType` is automatically cast to the correct concrete type. For example:

```
      c0 = arith.ConstantOp(f32, 0.0)
      # CHECK: F32Type(f32)
      print(repr(c0.result.type))

      unranked_tensor_type = UnrankedTensorType.get(f32)
      unranked_tensor = tensor.FromElementsOp(unranked_tensor_type, [c0]).result

      # CHECK: UnrankedTensorType
      print(type(unranked_tensor.type).__name__)
      # CHECK: UnrankedTensorType(tensor<*xf32>)
      print(repr(unranked_tensor.type))
```

This functionality immediately extends to typed attributes (i.e., `attr.type`).

The diff also implements similar functionality for `mlir_type_subclass`es but in a slightly different way - for such types (which have no cpp corresponding `class` or `struct`) the user must provide a type caster in python (similar to how `AttrBuilder` works) or in cpp as a `py::cpp_function`.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D150927

1 files changed, 2 insertions, 0 deletions

diff --git a/mlir/test/python/lib/PythonTestCAPI.h b/mlir/test/python/lib/PythonTestCAPI.h
index 90c5d4a383f9..5f1ed3a5b2ad 100644
--- a/mlir/test/python/lib/PythonTestCAPI.h
+++ b/mlir/test/python/lib/PythonTestCAPI.h
@@ -27,6 +27,8 @@ MLIR_CAPI_EXPORTED bool mlirTypeIsAPythonTestTestType(MlirType type);
 
 MLIR_CAPI_EXPORTED MlirType mlirPythonTestTestTypeGet(MlirContext context);
 
+MLIR_CAPI_EXPORTED MlirTypeID mlirPythonTestTestTypeGetTypeID(void);
+
 MLIR_CAPI_EXPORTED bool mlirTypeIsAPythonTestTestTensorValue(MlirValue value);
 
 #ifdef __cplusplus