Python: Improve workflow builder API to discourage inline executor instantiation

[TaoChenOSU]

The current workflow builder makes it very easy for users and AI to mistakenly instantiate executors inline. For example,

workflow = (
  WorkflowBuilder()
  .add_edge(AddOneExecutor(), AddOneExecutor())
  .add_edge(AddOneExecutor(), AggregateResultExecutor())
  .set_start_executor(AddOneExecutor())
  .build()
)

This is problematic because the nodes created are never connected the way users may expect them to.

A recommended way to improve the API to prevent this from happening:

builder = WorkflowBuilder()
add_one_executor = builder.register(AddOneExecutor(), name="AddOne")
aggregate_result_executor = builder.register(AggregateResultExecutor(), name="AggregateResult")

workflow = (
  builder
  .add_edge(add_one_executor, add_one_executor)
  .add_edge(add_one_executor, aggregate_result_executor)
  .set_start_executor(add_one_executor)
  .build()
)

[ekzhu]

Makes a lot of sense!!

[ekzhu]

The example code still breaks the builder into two places. How about:

builder = WorkflowBuilder()
add_one_executor = AddOneExecutor()
aggregate_result_executor = AggregateResultExecutor()

workflow = (
  builder
  .add_executor(add_on_executor, name="AddOneExecutor")
  .add_executor(aggregate_result_executor, name="AggregateResultExecutor")
  .add_edge(add_one_executor, add_one_executor)
  .add_edge(add_one_executor, aggregate_result_executor)
  .set_start_executor(add_one_executor)
  .build()
)

We just require that when adding edge, the source and target executor must be added already.

This way the builder code is all in one place.