Move LLM-related components into LLM module (#74)

* Move splitter into indexing module
* Rename post_processing module to parsers
* Migrate LLM-specific composite pipelines into llms module

This change moves the `splitters` module into `indexing` module. The `indexing` module will be created soon, to house `indexing`-related components.

This change renames `post_processing` module into `parsers` module. Post-processing is a generic term which provides very little information. In the future, we will add other extractors into the `parser` module, like Metadata extractor...

This change migrates the composite elements into `llms` module. These elements heavily assume that the internal nodes are llm-specific. As a result, migrating these elements into `llms` module will make them more discoverable, and simplify code base structure.

knowledgehub/llms/__init__.py

@@ -1,8 +1,9 @@
 from langchain.schema.messages import AIMessage, SystemMessage
 
-from .chats import AzureChatOpenAI, ChatLLM
-from .chats.base import BaseMessage, HumanMessage
+from .branching import GatedBranchingPipeline, SimpleBranchingPipeline
+from .chats import AzureChatOpenAI, BaseMessage, ChatLLM, HumanMessage
 from .completions import LLM, AzureOpenAI, OpenAI
+from .linear import GatedLinearPipeline, SimpleLinearPipeline
 from .prompts import BasePromptComponent, PromptTemplate
 
 __all__ = [
@@ -20,4 +21,9 @@ __all__ = [
     # prompt-specific components
     "BasePromptComponent",
     "PromptTemplate",
+    # strategies
+    "SimpleLinearPipeline",
+    "GatedLinearPipeline",
+    "SimpleBranchingPipeline",
+    "GatedBranchingPipeline",
 ]