Unit tests loading from pretrained fill missing keys

[richardkronick]

Description

Addition of unit tests covering the fill_missing_keys function from loading_from_pretrained.py.

Fixes # (issue)

Type of change

New unit tests.

Screenshots

Please attach before and after screenshots of the change if applicable.

Checklist:

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

[bryce13950]

Beautiful