[ROCm] [CI] Fix diffusion test

[tjtanaa]

PLEASE FILL IN THE PR DESCRIPTION HERE.

Purpose

Fix Diffusion GPU Worker Test
https://buildkite.com/vllm/vllm-omni-amd-ci/builds/8941/list?sid=019edeb5-8b47-4515-810d-ef3d20b13b1c&tab=output

    func = self.__getitem__(name)
           ^^^^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
self = <CDLL '/usr/local/lib/python3.12/dist-packages/tilelang/lib/libcudart_stub.so', handle 16de6880 at 0x7b88ec090da0>
name_or_ordinal = 'hipSetDevice'
    def __getitem__(self, name_or_ordinal):
>       func = self._FuncPtr((name_or_ordinal, self))
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       AttributeError: /usr/local/lib/python3.12/dist-packages/tilelang/lib/libcudart_stub.so: undefined symbol: hipSetDevice
/usr/lib/python3.12/ctypes/__init__.py:397: AttributeError
________ ERROR at setup of TestDiffusionWorkerSleep.test_sleep_level_2 _________
self = <test_diffusion_worker.TestDiffusionWorkerSleep object at 0x7b99413184d0>
mocker = <pytest_mock.plugin.MockerFixture object at 0x7b88ec091be0>
    @pytest.fixture(autouse=True)
    def setup_allocator(self, mocker: MockerFixture):
        """
        Unified interception of Allocators, and provision of default security values.
        """
>       self.mock_allocator_class = mocker.patch("vllm.device_allocator.cumem.CuMemAllocator")
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Test Plan

test-ready.yaml: https://buildkite.com/vllm/vllm-omni-amd-ci/builds/8942/list

The same test command is run on test-merge.yaml, so only test-ready.yaml results is sufficient.

vLLM Version:

vLLM-Omni Commit:

Test Result

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[hsliuustc0106]

Test-only fix looks correct. The _get_cumem_allocator_class() wrapper prevents hipSetDevice errors on ROCm by deferring the CuMemAllocator import. All CI checks pass.

[hsliuustc0106]

Good: Lazy import wrapper prevents hipSetDevice symbol error on ROCm.

[hsliuustc0106]

Good: Helper function centralizes the mock patching pattern.

[hsliuustc0106]

amd ci passed