Genai SA block integration

[vgrau98]

Fixes #7227

Description

Integrate the private SABlock from monai/networks/nets/transformer.py to monai/networks/blocks/selfattention.py

Add:

• causal self attention
• flash attention

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[vgrau98]

Duplicate with #7715