Added blog post "FlexAttention: The Flexibility of PyTorch with the Performance of FlashAttention" (#1701)

* Added blog post "FlexAttention: The Flexibility of PyTorch with the Performance of FlashAttention"

Signed-off-by: Chris Abraham <cjyabraham@gmail.com>

* Update publish date

Signed-off-by: Chris Abraham <cjyabraham@gmail.com>

* Update 2024-08-07-flexattention.md

* Update 2024-08-07-flexattention.md

---------

Signed-off-by: Chris Abraham <cjyabraham@gmail.com>
Co-authored-by: Kylie Wagar-Dirks <107439830+kyliewd@users.noreply.github.com>

Author: cjyabraham