Use C API to implement Operation._input_types

This change first converts _input_types into a property and renames the
member to _input_types_val. We keep _input_dtypes as an alias for
_input_types as it was before this change.

Similarly to _output_types, we can't enable normal tests yet. Instead,
we add a simple temporary test for _input_types.

Also, fix two minor typos in doc strings of function.py

PiperOrigin-RevId: 161597185