[CI/Build] Fix AMD CI: test_cpu_gpu.py

[zhewenl]

Purpose

test_cpu_gpu.py is added in #21448, while it add different attention backends to test, some backends might not be compatible with platforms - like FlashInferBackend is not supported in ROCM at the current moment.

This PR refactors the test to conditional import backends, like what we did in test_attention_backends.py.

Test Plan

pytest -v -s v1/kv_offload/test_cpu_gpu.py
INFO 10-22 21:15:25 [__init__.py:225] Automatically detected platform rocm.
================================================================= test session starts =================================================================
platform linux -- Python 3.12.11, pytest-8.4.2, pluggy-1.6.0 -- /home/zhewenli/uv_env/vllm-fork/bin/python3
cachedir: .pytest_cache
rootdir: /data/users/zhewenli/gitrepos/vllm-fork
configfile: pyproject.toml
plugins: anyio-4.11.0, asyncio-1.2.0
asyncio: mode=Mode.STRICT, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collecting ... WARNING 10-22 21:15:43 [interface.py:512] Current platform cuda does not have '__test__' attribute.
WARNING 10-22 21:15:43 [interface.py:512] Current platform cuda does not have '__bases__' attribute.
WARNING 10-22 21:15:43 [interface.py:512] Current platform cuda does not have '__test__' attribute.
collected 4 items                                                                                                                                     

v1/kv_offload/test_cpu_gpu.py::test_transfer[cuda:0-0-dtype0-4-256-64-1-16-8-64-3-True] INFO 10-22 21:15:43 [cpu_gpu.py:78] Allocating 4 CPU tensors...
PASSED
v1/kv_offload/test_cpu_gpu.py::test_transfer[cuda:0-0-dtype0-4-256-64-1-16-8-64-3-False] INFO 10-22 21:15:44 [cpu_gpu.py:78] Allocating 4 CPU tensors...
PASSED
v1/kv_offload/test_cpu_gpu.py::test_transfer[cuda:0-0-dtype0-4-256-64-3-16-8-64-3-True] INFO 10-22 21:15:44 [cpu_gpu.py:78] Allocating 4 CPU tensors...
PASSED
v1/kv_offload/test_cpu_gpu.py::test_transfer[cuda:0-0-dtype0-4-256-64-3-16-8-64-3-False] INFO 10-22 21:15:45 [cpu_gpu.py:78] Allocating 4 CPU tensors...
PASSED

================================================================== warnings summary ===================================================================
<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyPacked has no __module__ attribute

<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyObject has no __module__ attribute

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
============================================================ 4 passed, 2 warnings in 2.86s ============================================================

[DarkLight1337]

Let's use the previous logic for all non-ROCm platforms, to avoid changing the logic prior to this PR

[zhewenl]

@DarkLight1337 sounds good, updated - since I am working on checking AMD CI in general, I wonder if we want to use current_platform to gate CUDA specific kernels/features? The context is I found there are many tests with assumption that they will be running on CUDA platforms, thus these test are failing on AMD.

For example, ROCM uses TritonAttentionImpl as default, which doesn't support models with encoder(like openai/whisper)

[DarkLight1337]

cc @LucasWilkinson