[VectorDistribute] Add missing fills in attention tests

[sommerlukas]

#24110 updated tests to use linalg_ext.online_attention instead of linalg_ext.attention. In some tests, we directly use tensor.empty as outputs/accmulators for the attention. However, these should be initialized appropriately with linalg.fill.

Goal is to update all attention lit tests to use linalg.fill for accumulators/output.