Fix: Ensure model_id is a string when using a model from s3

[fadebek]

This PR fixes the following error:

Traceback (most recent call last):

  File "/opt/conda/bin/lorax-server", line 8, in <module>
    sys.exit(app())

  File "/opt/conda/lib/python3.10/site-packages/lorax_server/cli.py", line 89, in serve
    server.serve(

  File "/opt/conda/lib/python3.10/site-packages/lorax_server/server.py", line 299, in serve
    asyncio.run(

  File "/opt/conda/lib/python3.10/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)

  File "/opt/conda/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
    return future.result()

  File "/opt/conda/lib/python3.10/site-packages/lorax_server/server.py", line 251, in serve_inner
    model = get_model(

  File "/opt/conda/lib/python3.10/site-packages/lorax_server/models/__init__.py", line 138, in get_model
    if "facebook/galactica" in model_id:

TypeError: argument of type 'PosixPath' is not iterable

This error occurs when using a base model from s3

[tgaddair]

Thanks for the fix!