Remove large test data file from tracking

Add model_layers_0_self_attn_k_proj.pth (258 MB) to .gitignore
to comply with GitHub's 100 MB file size limit.
The regression tests already skip when this file is absent.

Made-with: Cursor

Author: FKKimura

.gitignore

@@ -16,3 +16,5 @@ site/
 debug_code/
 .pytest_cache/
 *.pt
+# Large test data (download separately if needed for regression tests)
+tests/onecomp/quantizer/jointq/data/model_layers_0_self_attn_k_proj.pth