Update paper.bib

[abhi-glitchhg]

added citations for papers.

[abhi-glitchhg]

In this PR, I've added citations for the following papers

• [x] Vanilla ViT
• [x] Swin Transformer
• [x] Pyramid Vision Transformer
• [x] CrossViT
• [x] Compact-Transformers
• [x] Visformer
• [x] Dense Prediction Transformer
• [x] CvT
• [x] convit
• [x] ViViT
• [x] perceiver-pytorch
• [x] memory-efficient-attention