Pydantic AI 的 Graph API 是如何定义工作流的？

from pydantic_graph import Graph, BaseNode, End
from dataclasses import dataclass

@dataclass
class ConversationState:
    user_name: str | None = None
    interests: list[str] = field(default_factory=list)

@dataclass
class GreetUser(BaseNode[ConversationState]):
    async def run(self, ctx: GraphRunContext[ConversationState]) -> 'AskName':
        print("Hello! Welcome to our recommendation system.")
        return AskName()

@dataclass
class AskName(BaseNode[ConversationState]):
    async def run(self, ctx: GraphRunContext[ConversationState]) -> 'AskInterests':
        name = input("What's your name? ")
        ctx.state.user_name = name
        return AskInterests()

# 定义图
conversation_graph = Graph(
    nodes=[GreetUser, AskName, AskInterests, GenerateRecommendations, SayGoodbye],
    state_type=ConversationState,
)