Enabled and fixed python type annotations in some modules in the executors package.

[copybara-service]

Enabled and fixed python type annotations in some modules in the executors package.

This revealed two Python typing-related issues:

1. Every subclass of executor_base.Executor was using a property on the corresponding subclass of executor_value_base.ExecutorValue without having access to that property. This was fixed by:
2. The methods on the subclasses of federating_executor.FederatingStrategy (FederatedComposingStrategy and FederatedResolvingStrategy) were annotated to return types specific to that subclass. This return type conflicted with the return types of the shared implementations defined in executor_utils.
3. The type annotation of target_executors parameter of FederatedResolvingStrategy was incorrect.

• Enabled pytype in the following modules:
  • federated_composing_strategy
  • federated_resolving_strategy
  • federating_executor
  • reference_resolving_executor
  • remote_executor
  • thread_delegating_executor
  • value_serialization

To fix issue #1:

• Promoted the reference property from the subclasses of executor_value_base.ExecutorValue to executor_value_base.ExecutorValue.
• Renamed usage of this API to be consistent.

To fix issue #2:

• Updated the methods on the subclasses of federating_executor.FederatingStrategy (FederatedComposingStrategy and FederatedResolvingStrategy) to accept and return executor_value_base.ExecutorValue types.

Note: That this does make the return types more generic, but I believe this is the intended interface. An alternative fix could be to update the executor_utils function to take and return TypeVar's (i.e. generic functions). However, I don't think this is the intended interface.

To fix issue #3:

• Updated the type annotation of the target_executors parameter to dict[placements.PlacementLiteral, Union[list[executor_base.Executor], executor_base.Executor]].