Switch cond_v2 to using tfe.defun instead of function.Defun.

This requires a few changes:
- Make func_graph_from_py_func public (not part of the official public API though)
- Make function_def_to_graph return the new FuncGraph implementation.
- Disables some cond_v2 tests until we get them working with the new FuncGraph implementation.
- Add outer_graph field to FuncGraph.
- Add external_captures and internal_captures properties to FuncGraph for readability.
- Remove extra_inputs/extra_args terminology from cond_v2_impl for readability.
- Use compat.as_str() around Graph._functions keys. In Python 3, we were somehow getting a mix of str and bytes objects.

PiperOrigin-RevId: 210015940