Fix GroupQueryAttention benchmark script (#20291)
### Description
Fix a few issues in GQA:
(1) memory efficient attention does not have bfloat16, need disable it
when bfloat16 is used.
(2) When prompt length is 1, it is not classified as prompt.
(3) Fix benchmark_gqa.py
(4) Add a comment about seqlen_k to avoid confusion.
### Motivation and Context
https://github.com/microsoft/onnxruntime/pull/20279
