[MLIR][Python] add value attr for PyAffineMapAttribute (#97254)

Similar to other attributes in Binding, the `PyAffineMapAttribute`
should include a value attribute to enable users to directly retrieve
the `AffineMap` from the `AffineMapAttr`.

Author: xurui1995

IRAttributes.cpp

@@ -142,6 +142,8 @@ class PyAffineMapAttribute : public PyConcreteAttribute<PyAffineMapAttribute> {
           return PyAffineMapAttribute(affineMap.getContext(), attr);
         },
         py::arg("affine_map"), "Gets an attribute wrapping an AffineMap.");
+    c.def_property_readonly("value", mlirAffineMapAttrGetValue,
+                            "Returns the value of the AffineMap attribute");
   }
 };