Removed deprecated llm attribute for load_chain (#5343)

# Removed deprecated llm attribute for load_chain

Currently `load_chain` for some chain types expect `llm` attribute to be
present but `llm` is deprecated attribute for those chains and might not
be persisted during their `chain.save`.

Fixes #5224
[(issue)](https://github.com/hwchase17/langchain/issues/5224)

## Who can review?
@hwchase17
@dev2049

---------

Co-authored-by: imeckr <chandanroutray2012@gmail.com>

langchain/chains/loading.py

@@ -138,19 +138,31 @@ def _load_map_reduce_documents_chain(
 
 
 def _load_llm_bash_chain(config: dict, **kwargs: Any) -> LLMBashChain:
-    if "llm" in config:
+    llm_chain = None
+    if "llm_chain" in config:
+        llm_chain_config = config.pop("llm_chain")
+        llm_chain = load_chain_from_config(llm_chain_config)
+    elif "llm_chain_path" in config:
+        llm_chain = load_chain(config.pop("llm_chain_path"))
+    # llm attribute is deprecated in favor of llm_chain, here to support old configs
+    elif "llm" in config:
         llm_config = config.pop("llm")
         llm = load_llm_from_config(llm_config)
+    # llm_path attribute is deprecated in favor of llm_chain_path,
+    # its to support old configs
     elif "llm_path" in config:
         llm = load_llm(config.pop("llm_path"))
     else:
-        raise ValueError("One of `llm` or `llm_path` must be present.")
+        raise ValueError("One of `llm_chain` or `llm_chain_path` must be present.")
     if "prompt" in config:
         prompt_config = config.pop("prompt")
         prompt = load_prompt_from_config(prompt_config)
     elif "prompt_path" in config:
         prompt = load_prompt(config.pop("prompt_path"))
-    return LLMBashChain(llm=llm, prompt=prompt, **config)
+    if llm_chain:
+        return LLMBashChain(llm_chain=llm_chain, prompt=prompt, **config)
+    else:
+        return LLMBashChain(llm=llm, prompt=prompt, **config)
 
 
 def _load_llm_checker_chain(config: dict, **kwargs: Any) -> LLMCheckerChain:
@@ -200,19 +212,31 @@ def _load_llm_checker_chain(config: dict, **kwargs: Any) -> LLMCheckerChain:
 
 
 def _load_llm_math_chain(config: dict, **kwargs: Any) -> LLMMathChain:
-    if "llm" in config:
+    llm_chain = None
+    if "llm_chain" in config:
+        llm_chain_config = config.pop("llm_chain")
+        llm_chain = load_chain_from_config(llm_chain_config)
+    elif "llm_chain_path" in config:
+        llm_chain = load_chain(config.pop("llm_chain_path"))
+    # llm attribute is deprecated in favor of llm_chain, here to support old configs
+    elif "llm" in config:
         llm_config = config.pop("llm")
         llm = load_llm_from_config(llm_config)
+    # llm_path attribute is deprecated in favor of llm_chain_path,
+    # its to support old configs
     elif "llm_path" in config:
         llm = load_llm(config.pop("llm_path"))
     else:
-        raise ValueError("One of `llm` or `llm_path` must be present.")
+        raise ValueError("One of `llm_chain` or `llm_chain_path` must be present.")
     if "prompt" in config:
         prompt_config = config.pop("prompt")
         prompt = load_prompt_from_config(prompt_config)
     elif "prompt_path" in config:
         prompt = load_prompt(config.pop("prompt_path"))
-    return LLMMathChain(llm=llm, prompt=prompt, **config)
+    if llm_chain:
+        return LLMMathChain(llm_chain=llm_chain, prompt=prompt, **config)
+    else:
+        return LLMMathChain(llm=llm, prompt=prompt, **config)
 
 
 def _load_map_rerank_documents_chain(
@@ -229,13 +253,22 @@ def _load_map_rerank_documents_chain(
 
 
 def _load_pal_chain(config: dict, **kwargs: Any) -> PALChain:
-    if "llm" in config:
+    llm_chain = None
+    if "llm_chain" in config:
+        llm_chain_config = config.pop("llm_chain")
+        llm_chain = load_chain_from_config(llm_chain_config)
+    elif "llm_chain_path" in config:
+        llm_chain = load_chain(config.pop("llm_chain_path"))
+    # llm attribute is deprecated in favor of llm_chain, here to support old configs
+    elif "llm" in config:
         llm_config = config.pop("llm")
         llm = load_llm_from_config(llm_config)
+    # llm_path attribute is deprecated in favor of llm_chain_path,
+    # its to support old configs
     elif "llm_path" in config:
         llm = load_llm(config.pop("llm_path"))
     else:
-        raise ValueError("One of `llm` or `llm_path` must be present.")
+        raise ValueError("One of `llm_chain` or `llm_chain_path` must be present.")
     if "prompt" in config:
         prompt_config = config.pop("prompt")
         prompt = load_prompt_from_config(prompt_config)
@@ -243,7 +276,10 @@ def _load_pal_chain(config: dict, **kwargs: Any) -> PALChain:
         prompt = load_prompt(config.pop("prompt_path"))
     else:
         raise ValueError("One of `prompt` or `prompt_path` must be present.")
-    return PALChain(llm=llm, prompt=prompt, **config)
+    if llm_chain:
+        return PALChain(llm_chain=llm_chain, prompt=prompt, **config)
+    else:
+        return PALChain(llm=llm, prompt=prompt, **config)
 
 
 def _load_refine_documents_chain(config: dict, **kwargs: Any) -> RefineDocumentsChain: