Runnable single protocol (#7800)

Objects implementing Runnable: BasePromptTemplate, LLM, ChatModel,
Chain, Retriever, OutputParser

- [x] Implement Runnable in base Retriever
- [x] Raise TypeError in operator methods for unsupported things 
- [x] Implement dict which calls values in parallel and outputs dict
with results
- [x] Merge in `+` for prompts
- [x] Confirm precedence order for operators, ideal would be `+` `|`,
https://docs.python.org/3/reference/expressions.html#operator-precedence
- [x] Add support for openai functions, ie. Chat Models must return
messages
- [x] Implement BaseMessageChunk return type for BaseChatModel, a
subclass of BaseMessage which implements __add__ to return
BaseMessageChunk, concatenating all str args
- [x] Update implementation of stream/astream for llm and chat models to
use new `_stream`, `_astream` optional methods, with default
implementation in base class `raise NotImplementedError` use
https://stackoverflow.com/a/59762827 to see if it is implemented in base
class
- [x] Delete the IteratorCallbackHandler (leave the async one because
people using)
- [x] Make BaseLLMOutputParser implement Runnable, accepting either str
or BaseMessage
---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/experimental/langchain_experimental/llms/llamaapi.py

@@ -9,10 +9,7 @@ from typing import (
     Tuple,
 )
 
-from langchain.callbacks.manager import (
-    AsyncCallbackManagerForLLMRun,
-    CallbackManagerForLLMRun,
-)
+from langchain.callbacks.manager import CallbackManagerForLLMRun
 from langchain.chat_models.base import BaseChatModel
 from langchain.schema import (
     ChatGeneration,
@@ -116,15 +113,6 @@ class ChatLlamaAPI(BaseChatModel):
             generations.append(gen)
         return ChatResult(generations=generations)
 
-    async def _agenerate(
-        self,
-        messages: List[BaseMessage],
-        stop: Optional[List[str]] = None,
-        run_manager: Optional[AsyncCallbackManagerForLLMRun] = None,
-        **kwargs: Any,
-    ) -> ChatResult:
-        raise NotImplementedError
-
     @property
     def _client_params(self) -> Mapping[str, Any]:
         """Get the parameters used for the client."""