* Fix MHAEinsum weight dimension bug when d_in != d_out (#857)
Previously MHAEinsum initialized weight matrices with shape (d_out, d_in) and used inappropriate einsum notation, causing failures for non-square input-output dimensions. This commit corrects weight initialization to shape (d_in, d_out), updates einsum notation to 'bnd,do->bno', and adds three unit tests to verify parity across different d_in and d_out settings. All tests pass successfully.
* use pytest
* Update .gitignore
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Co-authored-by: rasbt <mail@sebastianraschka.com>