1. Fix inconsistency between FunctionSpec of defun.Function and...

1. Fix inconsistency between FunctionSpec of defun.Function and function.Function by passing on FunctionSpec to a child defun.
2. Refactoring:
 - stop duplicating _input_signature and read from function_spec directly,
 - because _input_signature is gone, read from public property input_signature,
 - also make the input_signature of function.Function public to make it consistent. Some code expects to read the same attribute from both.
 - one corollary: we don't need to canonicalize ourselves any more.

PiperOrigin-RevId: 236873065