### What this PR does / why we need it?
Fix https://github.com/vllm-project/vllm-ascend/issues/5297, for
`AscendMMEncoderAttention` forward, we should keep the output shape
consistence with the input
- vLLM version: release/v0.13.0
- vLLM main:
81786c8774
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Signed-off-by: wangli <wangli858794774@gmail.com>