Worked on the facebookresearch/xformers repository to restore the BlockDiagonalMask import within the attention mechanism codebase. Addressed a bug where the BlockDiagonalMask import had been inadvertently removed, which risked breaking API compatibility and causing runtime errors for downstream models. The solution involved carefully reintroducing the import in xformers/ops/fmha/__init__.py, ensuring that existing models relying on this functionality continued to operate as expected. Utilized Python for the bug fix and applied code refactoring principles to maintain code clarity and stability. This work focused on preserving core functionality and minimizing disruption for users dependent on the attention mechanism’s BlockDiagonalMask feature.