fix deprecated paddle compat api usage

[ShigureNyako]

Summary

• replace paddle.compat.enable_torch_proxy(scope={"flashinfer"}) in README.md with paddle.enable_compat(scope={"flashinfer"})
• replace the remaining runtime calls in tests/conftest.py, tests/moe/test_trtllm_gen_fused_moe.py, and tests/attention/test_attention_sink_blackwell.py with paddle.enable_compat()
• update the stale commented example in tests/comm/test_trtllm_allreduce_fusion_paddle.py

Verification

• grep -R -nE "enable_torch_proxy|disable_torch_proxy" .
• python3 -m py_compile tests/conftest.py tests/moe/test_trtllm_gen_fused_moe.py tests/attention/test_attention_sink_blackwell.py tests/comm/test_trtllm_allreduce_fusion_paddle.py

[ShigureNyako]

已自查本 PR：

• 仅替换命中的旧 compat API 用法/注释，未做其他改动
• grep -R -nE "enable_torch_proxy|disable_torch_proxy" . 无残留
• python3 -m py_compile tests/conftest.py tests/moe/test_trtllm_gen_fused_moe.py tests/attention/test_attention_sink_blackwell.py tests/comm/test_trtllm_allreduce_fusion_paddle.py 通过

@SigureMo 麻烦 review，感谢！