Add CvT
# What does this PR do?

Co-authored-by: AnugunjNaman <anugunjjha@gmail.com>

This PR adds CvT (Convolutional Vision Transformer) by Microsoft Research.

I just cleaned up the branch of @AnugunjNaman. 

To do:

- [x] make @AnugunjNaman co-author of this PR
_The documentation is not available anymore as the PR was closed or merged._