Fix the device error when using ray as vllm-acend backend (#884)

1. Remove RAY_EXPERIMENTAL_NOSET_ASCEND_RT_VISIBLE_DEVICES
2. Add lazy init for vllm_ascend_C

Signed-off-by: zhuo97 <1103045176@qq.com>

examples/offline_multi_step_custom_ops.py

@@ -19,9 +19,6 @@
 
 from vllm import LLM, SamplingParams
 
-import vllm_ascend.platform as pf
-
-pf.CUSTOM_OP_ENABLED = True  # set True for custom Ops of Multi-Step.
 prompts = [
     "Hello, my name is",
     "The president of the United States is",