Add sleep mode feature for Ascend NPU (#513)

### What this PR does / why we need it?
This PR adds sleep mode feature for vllm-ascend, when sleeps, we do
mainly two things:

- offload model weights
- discard kv cache

RLHF tools(such as https://github.com/volcengine/verl and
https://github.com/OpenRLHF/OpenRLHF) have a strong need of sleep mode
to accelerate the training process.

This PR may solve #375 and #320 .

### Does this PR introduce _any_ user-facing change?
No existing user interfaces changed.
Users will have two new methods(`sleep()` and `wake_up()`) to use.

### How was this patch tested?
This PR is tested with Qwen/Qwen2.5-0.5B-Instruct.

At first, we have free NPU memory M1.

After `llm = LLM("Qwen/Qwen2.5-0.5B-Instruct", enable_sleep_mode=True)`
executed, we have free NPU memory M2. M2 < M1.

Then we call `llm.sleep(level=1)`, we have free NPU memory M3.

We have M3 > M2, M3 is very close to M1.

Plus, we have the same output tokens before sleep and after wake up,
with the config of `SamplingParams(temperature=0, max_tokens=10)` and
with the same input tokens of course.


This PR is utilizing the CMake procedure of #371 , thanks a lot.

Signed-off-by: Shuqiao Li <celestialli@outlook.com>

CMakeLists.txt

@@ -3,7 +3,7 @@ project(vllm_ascend_C)
 
 # include(CheckCXXcompilerFlag)
 # check_cxx_compiler_flag("-std=c++17", COMPILER_SUPPORTS_CXX17)
-
+set(CMAKE_CXX_STANDARD 17)
 
 include(${CMAKE_CURRENT_LIST_DIR}/cmake/utils.cmake)