[JAX] Implement an initial object API for colocated Python

[copybara-service]

[JAX] Implement an initial object API for colocated Python

Colocated Python adds colocated_python_class. This API wraps a user-defined
class for automatic remoting of object construction/destruction and method calls:

• An object will be initialized on the backend. At least for now,
  initialization is deferred until the first method is called; at this point,
  colocated Python knows what devices the objects should be accessible and thus
  it can construct the object(s).

• When an object method is called, the method call runs as a colocated Python
  function call on the backend.

• When the object is destroyed (either by reaching a zero reference count or
  through Python GC), destruction also runs as a colocated Python function call
  and destroys all objects from the backend.

This change provides an intial API implementation. Main limitations are as
follows:

• The methods of a colocated Python class does not support specialization.
  Calling it requires at least one argument.

• Colocated Python objects cannot reference or interact with each other on the
  controller or on the colocated Python backend.

These limitations will be lifted as the object API implementation is improved.