Fix/chatgpt loader bug

[Yukimagi]

Description

This PR fixes a bug in ChatGPTLoader where the last message in the conversation was being dropped when using the default initialization arguments.

The Bug:
The __init__ method had a default value of num_logs=-1. In the load method, the code used slicing data[:self.num_logs] when num_logs was truthy. Since -1 is truthy, this resulted in data[:-1], which excluded the last element of the list.

The Fix:
Changed the default value of num_logs to 0 (which implies loading all logs), ensuring the slicing logic correctly handles the default case.

Testing:

• Added a new test file tests/unit_tests/document_loaders/test_chatgpt.py.
• Added unit tests to verify correct loading of conversation data.
• Verified that num_logs parameter works as expected.
• Coverage: Increased code coverage for chatgpt.py from 0% to 100%.

Issue

Fixes #465

Dependencies

None