Improvements to function._FuncGraph.

* Adds 'inputs', 'outputs', and 'name' field to _FuncGraph. This
  allows _FuncGraph to encapsulate all the information needed to
  convert it to a FunctionDef.
* Refactor logic for converting a Python callable to a _FuncGraph into
  a new method, func_graph_from_py_func().

These changes are in preparation for converting tf.cond to emit an If
op. By exposing _FuncGraph functionality outside of _DefinedFunction,
_FuncGraphs can be used to represent functions that are manipulated
(e.g. to output intermediate tensors) before being converted to
FunctionDef protos.

PiperOrigin-RevId: 197003496