Adding CLS token back to Eva Transformer #14

It would be great to have the CLS token added back to the Eva Transformer and could be done conditionally based on a flag in the class def. https://docs.monai.io/en/1.3.0/_modules/monai/networks/nets/vit.html

Reason: Making Eva (and Primus by extension) compatible for pretraining with DINOv2 and for classification in general.