[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>

csrc/ops.h

@@ -46,4 +46,16 @@ namespace vllm_ascend {
     auto new_tensor = at_npu::native::from_blob(data_ptr, sizes, strides, options);
     return new_tensor;
   }
+    extern void launch_advance_step_flashattn(
+        void* stream,
+        int64_t num_seqs,
+        int64_t num_queries,
+        int64_t block_size,
+        int64_t* input_tokens_ptr,
+        int64_t* sampled_token_ids_ptr,
+        int64_t* input_positions_ptr,
+        int32_t* seq_lens_ptr,
+        int32_t* slot_mapping_ptr,
+        int32_t* block_tables_ptr,
+        int64_t block_tables_stride);
 }