Add Enum for agent types (#2321)

This pull request adds an enum class for the various types of agents
used in the project, located in the `agent_types.py` file. Currently,
the project is using hardcoded strings for the initialization of these
agents, which can lead to errors and make the code harder to maintain.
With the introduction of the new enums, the code will be more readable
and less error-prone.

The new enum members include:

- ZERO_SHOT_REACT_DESCRIPTION
- REACT_DOCSTORE
- SELF_ASK_WITH_SEARCH
- CONVERSATIONAL_REACT_DESCRIPTION
- CHAT_ZERO_SHOT_REACT_DESCRIPTION
- CHAT_CONVERSATIONAL_REACT_DESCRIPTION

In this PR, I have also replaced the hardcoded strings with the
appropriate enum members throughout the codebase, ensuring a smooth
transition to the new approach.

tests/unit_tests/agents/test_agent.py

@@ -5,6 +5,7 @@ from typing import Any, List, Mapping, Optional
 from pydantic import BaseModel
 
 from langchain.agents import AgentExecutor, initialize_agent
+from langchain.agents.agent_types import AgentType
 from langchain.agents.tools import Tool
 from langchain.callbacks.base import CallbackManager
 from langchain.llms.base import LLM
@@ -55,7 +56,11 @@ def _get_agent(**kwargs: Any) -> AgentExecutor:
         ),
     ]
     agent = initialize_agent(
-        tools, fake_llm, agent="zero-shot-react-description", verbose=True, **kwargs
+        tools,
+        fake_llm,
+        agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION,
+        verbose=True,
+        **kwargs,
     )
     return agent
 
@@ -98,7 +103,7 @@ def test_agent_with_callbacks_global() -> None:
     agent = initialize_agent(
         tools,
         fake_llm,
-        agent="zero-shot-react-description",
+        agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION,
         verbose=True,
         callback_manager=manager,
     )
@@ -144,7 +149,7 @@ def test_agent_with_callbacks_local() -> None:
     agent = initialize_agent(
         tools,
         fake_llm,
-        agent="zero-shot-react-description",
+        agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION,
         verbose=True,
         callback_manager=manager,
     )
@@ -191,7 +196,7 @@ def test_agent_with_callbacks_not_verbose() -> None:
     agent = initialize_agent(
         tools,
         fake_llm,
-        agent="zero-shot-react-description",
+        agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION,
         callback_manager=manager,
     )
 
@@ -223,7 +228,7 @@ def test_agent_tool_return_direct() -> None:
     agent = initialize_agent(
         tools,
         fake_llm,
-        agent="zero-shot-react-description",
+        agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION,
     )
 
     output = agent.run("when was langchain made")
@@ -249,7 +254,7 @@ def test_agent_tool_return_direct_in_intermediate_steps() -> None:
     agent = initialize_agent(
         tools,
         fake_llm,
-        agent="zero-shot-react-description",
+        agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION,
         return_intermediate_steps=True,
     )
 
@@ -280,7 +285,7 @@ def test_agent_with_new_prefix_suffix() -> None:
     agent = initialize_agent(
         tools=tools,
         llm=fake_llm,
-        agent="zero-shot-react-description",
+        agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION,
         agent_kwargs={"prefix": prefix, "suffix": suffix},
     )
 
@@ -307,7 +312,7 @@ def test_agent_lookup_tool() -> None:
     agent = initialize_agent(
         tools=tools,
         llm=fake_llm,
-        agent="zero-shot-react-description",
+        agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION,
     )
 
     assert agent.lookup_tool("Search") == tools[0]