feat: Add docstrings; Add Support for UserDict in _transfer_to_device

[proshian]

This PR

• Docstrings: Adds docstring to undocumented methods in base.py
• torch-influence & transformers: Updates the _transfer_to_device method to support UserDict instances. The Hugging Face BatchEncoding class, which is a common otype for a batch in the library, inherits from UserDict but not from dict. This change ensures that _transfer_to_device correctly handles these batch types, fixing error-raising in _transfer_to_device that prevented torch-influence from working seamlessly with the transformers library.