Move out pretrained weight conversion functions

[richardkronick]

Description

Moving all of the weight conversion functions from loading_from_pretrained.py into their own individual files to improve manageability. Also included one unit test to ensure that the functions are accessible. Future unit tests will be added.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Screenshots

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

Checklist:

• [ ] 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
• [x] 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]

One more needs to be moved out, which was added since the work on this started. There is one called t5 that you will see after you pull down the most recent changes on your branch.

[richardkronick]

One more needs to be moved out, which was added since the work on this started. There is one called t5 that you will see after you pull down the most recent changes on your branch.

convert_t5_weights has been moved to its own file