Add flag to enable running softmax in fp32 #71
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When run with VLLM_FP32_SOFTMAX=true both prompt attention (if FSDPA is disabled) and flat_pa will keep mme accumulated values from Q@K in fp32 and run softmax normalization in fp32 which helps with numerical errors in certain models like Qwen2.
This code should have no effect if the feature is disabled.