[ENG-586] Moved imports of langchain to speed up rasa --help (#12930)

* moved imports of langchain

* improve imports

rasa/shared/utils/llm.py

@@ -1,17 +1,17 @@
-from typing import Any, Dict, Optional, Text, Type
+from typing import Any, Dict, Optional, Text, Type, TYPE_CHECKING
 import warnings
 
 import structlog
-from langchain.embeddings.base import Embeddings
-from langchain.llms.base import BaseLLM
-from langchain.llms.loading import load_llm_from_config
-from langchain.cache import SQLiteCache
 
 from rasa.shared.core.trackers import DialogueStateTracker
 from rasa.shared.core.events import BotUttered, UserUttered
 from rasa.shared.engine.caching import get_local_cache_location
 import rasa.shared.utils.io
 
+if TYPE_CHECKING:
+    from langchain.embeddings.base import Embeddings
+    from langchain.llms.base import BaseLLM
+
 
 structlogger = structlog.get_logger()
 
@@ -122,6 +122,7 @@ def combine_custom_and_default_config(
 def ensure_cache() -> None:
     """Ensures that the cache is initialized."""
     import langchain
+    from langchain.cache import SQLiteCache
 
     # ensure the cache directory exists
     cache_location = get_local_cache_location()
@@ -133,7 +134,7 @@ def ensure_cache() -> None:
 
 def llm_factory(
     custom_config: Optional[Dict[str, Any]], default_config: Dict[str, Any]
-) -> BaseLLM:
+) -> "BaseLLM":
     """Creates an LLM from the given config.
 
     Args:
@@ -144,6 +145,8 @@ def llm_factory(
     Returns:
     Instantiated LLM based on the configuration.
     """
+    from langchain.llms.loading import load_llm_from_config
+
     ensure_cache()
 
     config = combine_custom_and_default_config(custom_config, default_config)
@@ -165,7 +168,7 @@ def llm_factory(
 
 def embedder_factory(
     custom_config: Optional[Dict[str, Any]], default_config: Dict[str, Any]
-) -> Embeddings:
+) -> "Embeddings":
     """Creates an Embedder from the given config.
 
     Args:
@@ -176,6 +179,7 @@ def embedder_factory(
     Returns:
     Instantiated Embedder based on the configuration.
     """
+    from langchain.embeddings.base import Embeddings
     from langchain.embeddings import (
         CohereEmbeddings,
         HuggingFaceHubEmbeddings,