Fix key and value heads patching for models with different n_heads from n_key_value_heads

[nikolaystanishev]

Description

• When patch key or value model heads n_key_value_heads is used instead of n_heads.
• When stacking the results for them in the attention head patching methods their results are padded due to different dimentions.

The problem is described in the corresponding issue.

Fixes #980

Type of change

Please delete options that are not relevant.

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

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
• 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