Fixing docstrings for cvt
# What does this PR do?

This PR does the following:
1. Remove the error in `README.md` where `CvT` description was copy of `CTRL`
2. Fix `size` of image for `feature extractor` which was set to `224`.
3. fix the input docstring for forward classes of `CvtModel` and `CvtForImageClassification` (head mask etc not needed).
4. Adding the largest `CvT` model. 

TODO: **Transfer the largest sized (1GB) model to Microsoft from anugunj/cvt-w24-384-22k**

@NielsRogge @LysandreJik 
_The documentation is not available anymore as the PR was closed or merged._