Fix CPU Attention overflow issue (#27822)
### Description
<!-- Describe your changes. -->
Fix `int` overflow issue in
`ComputeAttentionSoftmaxInplace<MLFloat16>()` by using `size_t` and `SafeInt` instead.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Fix overflow issue.
---------
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
