Allow running heterogeneous Gemma4 models on transformer v5.15.0

[jlarson4]

Description

Resolves issue with running heterogeneous Gemma4 models when on transformers v5.15.0 or later

Fixes #1647

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

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