【Hackathon 9th No.47】Unit test for append_attention

[cloudforge1]

Add unit tests for the append_attention op.

Source: look for append_attention in custom_ops/gpu_ops/
Registration: custom_ops/gpu_ops/cpp_extensions.cc
Test file: tests/operators/test_append_attention.py

Key attention mechanism op. Tests should verify correctness against a naive attention implementation, with different sequence lengths, head counts, and head dims.

Branch: task/047-append-attention-test