What this PR does / why we need it?

Based on the design of dual-batch overlap proposed by Deepseek team and also the implementation of fused moe in VLLM project, we implement the multi-stream(also known as dual-batch) overlap for deepseek+mla on Ascend NPU. We split the input batch of model into two microbatches and then overlap the comp/comm ops in attention and moe layers using two streams to improve the performance. Our approach can be easily extended when adding dispatch/combine communications for moe layer.
Compared with the previously proposed draft, we use one stream for computation ops and the other for communication ops, separately. In out opinions, it is beneficial for arranging the order of executing different ops and thus avoiding the contention of computation/communication resources.

Note that this PR is in progress. The benchmark performance will be updated soon.

ref: overlap for llama
ref: dbo in sglang

Does this PR introduce any user-facing change?

Adding an env variable "VLLM_ENABLE_DBO". Users can enable dbo by setting "VLLM_ENABLE_DBO=1"

How was this patch tested?

This patch can be tested with vllm-0.8.5 using its online service with benchmark tests. We have decoupled the func of dbo from vllm and it should be able to run without any modification to the code of vllm(some modifications is better to implement in vllm though).

Any advice/discussion is welcome.

Performance Benchmark

We have ran the benchmark_serving script of vllm to test the performance after using dual-batch overlap.
--dataset-name random --random-input-len 4096 --random-output-len 1 --num-prompts 20 --max-concurrency 8 --request-rate 5 --metric-percentiles 90

1. test with the version not using alltoall in Ascend A2 (tp16 ep16 + deepseek r1 w8a8)

prefill qps: 2.17-> 2.60

2. test with the version using alltoall (can be further optimized ):
   

prefill qps: 0.90 -> 1.01
Mean TTFT：8226->7432ms