Updated boot_transformers to use local hf_config, if provided

[jlarson4]

Description

When a local hf_model is provided, default to using that models hf_config object rather than requesting from HuggingFace again. This saves a network call to HF and will working better for users with limited access.

Added tests to confirm this new load path works as expected.

Fixes #846

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility