[mlir] Nits on uses of llvm::raw_string_ostream (NFC)

* Strip calls to raw_string_ostream::flush(), which is essentially a no-op
* Strip unneeded calls to raw_string_ostream::str(), to avoid excess indirection.

Author: JOE1994

mlir/lib/Bindings/Python/IRAttributes.cpp

@@ -708,7 +708,7 @@ class PyDenseElementsAttribute
         llvm::raw_string_ostream os(message);
         os << "Expected a static ShapedType for the shaped_type parameter: "
            << py::repr(py::cast(*explicitType));
-        throw py::value_error(os.str());
+        throw py::value_error(message);
       }
       shapedType = *explicitType;
     } else {
@@ -732,7 +732,7 @@ class PyDenseElementsAttribute
         os << "All attributes must be of the same type and match "
            << "the type parameter: expected=" << py::repr(py::cast(shapedType))
            << ", but got=" << py::repr(py::cast(attrType));
-        throw py::value_error(os.str());
+        throw py::value_error(message);
       }
     }