Add missing __init__.py to llms/providers

[zhudotexe]

Currently, trying to import evaluation_harness from another package causes the following error:

>>> from evaluation_harness import evaluator_router
Traceback (most recent call last):
  [...snip]
  File ".../venv/lib/python3.10/site-packages/evaluation_harness/helper_functions.py", line 18, in <module>
    from llms.providers.openai_utils import (
  File ".../venv/lib/python3.10/site-packages/llms/__init__.py", line 2, in <module>
    from .providers.hf_utils import generate_from_huggingface_completion
ModuleNotFoundError: No module named 'llms.providers'

This seems to be because llms/providers is missing an __init__.py file. This PR adds an empty __init__.py so that Python recognizes the submodule.