Average only valid part of the ipg buffer. (#5268)
When contiguous gradients is used ipg buffer may not be fully utilized.
Call average_tensor only for the slice with valid gradints
Change-Id: I760559d52c2f91e15cd6cd0b48e534ec2352802a
Co-authored-by: Logan Adams <114770087+loadams@users.noreply.github.com>
