DeepSpeed

ca4449e8 - Update version.txt after 0.15.0 release (#6403)

Update version.txt after 0.15.0 release (#6403) **Auto-generated PR to update version.txt after a DeepSpeed release** Released version - 0.15.0 Author - @loadams Co-authored-by: loadams <loadams@users.noreply.github.com>