CI fails for slow tests: ImportError: vLLM is not installed

[albertvillanova]

CI fails for slow tests: https://github.com/huggingface/trl/actions/runs/18522980131/job/52787020203

ImportError: vLLM is not installed. Please install it with pip install trl[vllm]

FAILED tests/test_vllm_client_server.py::TestVLLMClientServerDeviceParameter::test_init_communicator_with_device_int - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
FAILED tests/test_vllm_client_server.py::TestVLLMClientServerDeviceParameter::test_init_communicator_with_device_string - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
FAILED tests/test_vllm_client_server.py::TestVLLMClientServerDeviceParameter::test_init_communicator_with_torch_device - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
ERROR tests/test_vllm_client_server.py::TestVLLMClientServer::test_generate - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
ERROR tests/test_vllm_client_server.py::TestVLLMClientServer::test_generate_with_params - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
ERROR tests/test_vllm_client_server.py::TestVLLMClientServer::test_update_model_params - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
ERROR tests/test_vllm_client_server.py::TestVLLMClientServer::test_reset_prefix_cache - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
ERROR tests/test_vllm_client_server.py::TestVLLMClientServerBaseURL::test_generate - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
ERROR tests/test_vllm_client_server.py::TestVLLMClientServerBaseURL::test_generate_with_params - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
ERROR tests/test_vllm_client_server.py::TestVLLMClientServerBaseURL::test_update_model_params - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.
ERROR tests/test_vllm_client_server.py::TestVLLMClientServerBaseURL::test_reset_prefix_cache - ImportError: vLLM is not installed. Please install it with `pip install trl[vllm]`.