Expose BFloat in Python bindings (#8255)

There are two parts to support for BFloat16 in Python:
1) Ability to define kernels and AOT compile them [fixed in this PR]
2) Ability to call kernels from Python

This fixes part 1, which is what I need for my use case.  Part 2 is
blocked on bfloat16 support in Python buffer protocols. See #6849 for
more details.

python_bindings/src/halide/halide_/PyType.cpp

@@ -32,6 +32,9 @@ std::string halide_type_to_string(const Type &type) {
         case halide_type_float:
             stream << "float";
             break;
+        case halide_type_bfloat:
+            stream << "bfloat";
+            break;
         case halide_type_handle:
             if (type.handle_type) {
                 stream << type.handle_type->inner_name.name;
@@ -67,6 +70,7 @@ void define_type(py::module &m) {
         .def("is_vector", &Type::is_vector)
         .def("is_scalar", &Type::is_scalar)
         .def("is_float", &Type::is_float)
+        .def("is_bfloat", &Type::is_bfloat)
         .def("is_int", &Type::is_int)
         .def("is_uint", &Type::is_uint)
         .def("is_handle", &Type::is_handle)
@@ -94,6 +98,7 @@ void define_type(py::module &m) {
     m.def("Int", Int, py::arg("bits"), py::arg("lanes") = 1);
     m.def("UInt", UInt, py::arg("bits"), py::arg("lanes") = 1);
     m.def("Float", Float, py::arg("bits"), py::arg("lanes") = 1);
+    m.def("BFloat", BFloat, py::arg("bits"), py::arg("lanes") = 1);
     m.def("Bool", Bool, py::arg("lanes") = 1);
     m.def("Handle", make_handle, py::arg("lanes") = 1);