Add Convolutional vision Transformer (CvT)

[fffffgggg54]

CvT as described in https://arxiv.org/abs/2103.15808

Swin-era heirarchical transformer. From-scratch reimplementation, cleaner than original that exposes most module cfgs as kwargs, uses sdpa/timm style (https://github.com/microsoft/CvT/tree/main). WIP/barebones test for now, stuck at successful weight remap but incorrect activations that seem to come, at least in part, from BatchNorm layers.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[fffffgggg54]

Validation for cvt-13 lines up with paper (81.678 top-1 for me), acts are off from reference impl by minute amounts (MSE of logits for 1 sample off on the order of 1e-10). Initial problems had to do with norm before attn and attn residual when there is a cls_token. I'll finish this off later today most likely.