Fix Lark import error (#4421)

Any import that touches langchain.retrievers currently requires Lark.
Here's one attempt to fix. Not very pretty, very open to other ideas.
Alternatives I thought of are 1) make Lark requirement, 2) put
everything in parser.py in the try/except. Neither sounds much better

Related to #4316, #4275

langchain/chains/query_constructor/parser.py

@@ -10,7 +10,12 @@ try:
         )
     from lark import Lark, Transformer, v_args
 except ImportError:
-    pass
+
+    def v_args(*args: Any, **kwargs: Any) -> Any:  # type: ignore
+        return lambda _: None
+
+    Transformer = object  # type: ignore
+    Lark = object  # type: ignore
 
 from langchain.chains.query_constructor.ir import (
     Comparator,