[Performance]: Custom AscendC Kernel of Multi-Step Prepare Input (#814)

### What this PR does / why we need it?

- According to https://github.com/vllm-project/vllm-ascend/issues/807,
we pull request for customer ascendc kernel of multi-step.
- also a bug we found in multi_step_runner.py is fixed when we use
multi-step on V0 Engine.


### Does this PR introduce _any_ user-facing change?

no user-facing change


### How was this patch tested?
we add Unit Test file and offline inference file to test the custom
ascendc kernel. See test/ops/test_multi_step.py and
examples/offline_multi_step.py

---------

Signed-off-by: wan_danfeng <wonderful199082@126.com>

vllm_ascend/worker/multi_step_runner.py

@@ -14,7 +14,6 @@ from vllm.model_executor.layers.sampler import (PromptLogprobs, SampleLogprobs,
 from vllm.model_executor.model_loader.tensorizer import TensorizerConfig
 from vllm.sequence import (CompletionSequenceGroupOutput, IntermediateTensors,
                            Logprob, SequenceGroupMetadata, SequenceOutput)
-from vllm.utils import current_stream
 from vllm.worker.model_runner_base import (
     _init_attn_metadata_from_tensor_dict,
     _init_frozen_model_input_from_tensor_dict,
@@ -23,6 +22,7 @@ from vllm.worker.multi_step_model_runner import (ModelOutput,
                                                  PythonizationCache,
                                                  StatefulModelInput)
 
+from vllm_ascend.utils import current_stream
 from vllm_ascend.worker.model_runner import (
     ModelInputForNPUWithSamplingMetadata, NPUModelRunnerBase)