community: better support of pathlib paths in document loaders (#18396)

So this arose from the
https://github.com/langchain-ai/langchain/pull/18397 problem of document
loaders not supporting `pathlib.Path`.

This pull request provides more uniform support for Path as an argument.
The core ideas for this upgrade: 
- if there is a local file path used as an argument, it should be
supported as `pathlib.Path`
- if there are some external calls that may or may not support Pathlib,
the argument is immidiately converted to `str`
- if there `self.file_path` is used in a way that it allows for it to
stay pathlib without conversion, is is only converted for the metadata.

Twitter handle: https://twitter.com/mwmajewsk

libs/community/langchain_community/document_loaders/assemblyai.py

@@ -1,7 +1,8 @@
 from __future__ import annotations
 
 from enum import Enum
-from typing import TYPE_CHECKING, Iterator, Optional
+from pathlib import Path
+from typing import TYPE_CHECKING, Iterator, Optional, Union
 
 import requests
 from langchain_core.documents import Document
@@ -44,7 +45,7 @@ class AssemblyAIAudioTranscriptLoader(BaseLoader):
 
     def __init__(
         self,
-        file_path: str,
+        file_path: Union[str, Path],
         *,
         transcript_format: TranscriptFormat = TranscriptFormat.TEXT,
         config: Optional[assemblyai.TranscriptionConfig] = None,
@@ -71,7 +72,7 @@ class AssemblyAIAudioTranscriptLoader(BaseLoader):
         if api_key is not None:
             assemblyai.settings.api_key = api_key
 
-        self.file_path = file_path
+        self.file_path = str(file_path)
         self.transcript_format = transcript_format
         self.transcriber = assemblyai.Transcriber(config=config)