Update Cohere Reranker (#4180)

The forward ref annotations don't get updated if we only iimport with
type checking

---------

Co-authored-by: Abhinav Verma <abhinav_win12@yahoo.co.in>

langchain/retrievers/document_compressors/cohere_rerank.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING, Dict, Sequence
 
-from pydantic import root_validator
+from pydantic import Extra, root_validator
 
 from langchain.retrievers.document_compressors.base import BaseDocumentCompressor
 from langchain.schema import Document
@@ -10,6 +10,13 @@ from langchain.utils import get_from_dict_or_env
 
 if TYPE_CHECKING:
     from cohere import Client
+else:
+    # We do to avoid pydantic annotation issues when actually instantiating
+    # while keeping this import optional
+    try:
+        from cohere import Client
+    except ImportError:
+        pass
 
 
 class CohereRerank(BaseDocumentCompressor):
@@ -17,7 +24,13 @@ class CohereRerank(BaseDocumentCompressor):
     top_n: int = 3
     model: str = "rerank-english-v2.0"
 
-    @root_validator()
+    class Config:
+        """Configuration for this pydantic object."""
+
+        extra = Extra.forbid
+        arbitrary_types_allowed = True
+
+    @root_validator(pre=True)
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that api key and python package exists in environment."""
         cohere_api_key = get_from_dict_or_env(

tests/integration_tests/retrievers/document_compressors/test_cohere_reranker.py

@@ -0,0 +1,8 @@
+"""Test the cohere reranker."""
+
+from langchain.retrievers.document_compressors.cohere_rerank import CohereRerank
+
+
+def test_cohere_reranker_init() -> None:
+    """Test the cohere reranker initializes correctly."""
+    CohereRerank()