Remove einsum in apply_causal_mask in abstract_attention.py

[degenfabian]

Description

There are known accuracy issues which are to some extent caused by the usage of einsum across large parts of the codebase. This PR removes one of these usages in the create_alibi_bias function. There is no specific issue associated with this PR, although removing a lot of the einsum usages could help in removing implementation inaccuracies addressed in many issues. For this specific einsum removal I have not added any tests, since the specific if clause that was modified is called over 230 times over the entire test suite and all tests still pass. I am happy to write additional tests though if that would be helpful.

Type of change

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

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• 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