fix gqa cpu nan bug (#20521)
### Description
There was a bug with gqa on cpu where on token case, with batch_size >
1, and with past_present_share_buffer off, the output would occasionally
contain nans. this pr fixes that. it also updates documentation and
fixes posid gen for rotary in cuda in prompt case.
### Motivation and Context
this pr solves the GQA CPU bug as well as updates the documentation and
makes seqlens_k irrelevant for prompt case, which is useful to prevent
user error.
