support graph (#1152)

### What problem does this PR solve?

#918 
### Type of change

- [x] New Feature (non-breaking change which adds functionality)

graph/component/__init__.py

@@ -0,0 +1,16 @@
+import importlib
+from .begin import Begin, BeginParam
+from .generate import Generate, GenerateParam
+from .retrieval import Retrieval, RetrievalParam
+from .answer import Answer, AnswerParam
+from .categorize import Categorize, CategorizeParam
+from .switch import Switch, SwitchParam
+from .relevant import Relevant, RelevantParam
+from .message import Message, MessageParam
+from .rewrite import RewriteQuestion, RewriteQuestionParam
+
+
+def component_class(class_name):
+    m = importlib.import_module("graph.component")
+    c = getattr(m, class_name)
+    return c