[XLA:Python] Refactor jit argument parsing code.
[XLA:Python] Refactor jit argument parsing code.

* Put the logic to split static and dynamic arguments under ParseArguments, which returns a ArgumentSignature and a list of dynamic arguments.
* Move the other argument parsing logic for jit and pmap into a separate CallSignature type.

This simplifies the code, and prepares for adding another use case for the argument parsing code without the rest of the jit logic.

Refactoring only, no functional changes intended.
