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/pipelines/ingest.py

@@ -9,6 +9,7 @@ from theflow.utils.modules import ObjectInitDeclaration as _
 
 from kotaemon.base import BaseComponent
 from kotaemon.embeddings import AzureOpenAIEmbeddings
+from kotaemon.indexing.splitters import SimpleNodeParser
 from kotaemon.loaders import (
     AutoReader,
     DirectoryReader,
@@ -16,7 +17,6 @@ from kotaemon.loaders import (
     OCRReader,
     PandasExcelReader,
 )
-from kotaemon.parsers.splitter import SimpleNodeParser
 from kotaemon.pipelines.agents import BaseAgent
 from kotaemon.pipelines.indexing import IndexVectorStoreFromDocumentPipeline
 from kotaemon.pipelines.reranking import BaseRerankingPipeline