langchain[patch]: Improve HyDe with custom prompts and ability to supply the run_manager (#14016)

- **Description:** The class allows to only select between a few
predefined prompts from the paper. That is not ideal, since other use
cases might need a custom prompt. The changes made allow for this. To be
able to monitor those, I also added functionality to supply a custom
run_manager.
  - **Issue:** no issue, but a new feature,
  - **Dependencies:** none,
  - **Tag maintainer:** @hwchase17,
  - **Twitter handle:** @yvesloy

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/chains/hyde/base.py

@@ -9,6 +9,7 @@ from typing import Any, Dict, List, Optional
 import numpy as np
 from langchain_core.embeddings import Embeddings
 from langchain_core.language_models import BaseLanguageModel
+from langchain_core.prompts import BasePromptTemplate
 from langchain_core.pydantic_v1 import Extra
 
 from langchain.callbacks.manager import CallbackManagerForChainRun
@@ -72,11 +73,21 @@ class HypotheticalDocumentEmbedder(Chain, Embeddings):
         cls,
         llm: BaseLanguageModel,
         base_embeddings: Embeddings,
-        prompt_key: str,
+        prompt_key: Optional[str] = None,
+        custom_prompt: Optional[BasePromptTemplate] = None,
         **kwargs: Any,
     ) -> HypotheticalDocumentEmbedder:
-        """Load and use LLMChain for a specific prompt key."""
+        """Load and use LLMChain with either a specific prompt key or custom prompt."""
+        if custom_prompt is not None:
+            prompt = custom_prompt
+        elif prompt_key is not None and prompt_key in PROMPT_MAP:
             prompt = PROMPT_MAP[prompt_key]
+        else:
+            raise ValueError(
+                f"Must specify prompt_key if custom_prompt not provided. Should be one "
+                f"of {list(PROMPT_MAP.keys())}."
+            )
+
         llm_chain = LLMChain(llm=llm, prompt=prompt)
         return cls(base_embeddings=base_embeddings, llm_chain=llm_chain, **kwargs)