### What this PR does / why we need it?
Fix the precision issue in dispatch_ffn_combine_bf16 operator.
Remove redundant synchronization operations in dispatch_ffn_combine
operator.
- vLLM version: v0.16.0
- vLLM main:
4034c3d32e
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Signed-off-by: guanguan0308 <1546542263@qq.com>