Support New Methods and Update Documents (V0.2.6)

[Lupin1998]

Supporting new features:

1. Support new backbone architectures (EdgeNeXt, EfficientFormer, HorNet, MViT.V2, ShuffleNet.V1) and provide relevant network modules in models/utils/layers. Config files and README.md are updated.
2. Fix the updated arXiv.V2 version of VAN by adding architecture configurations.
3. Support ArcFace loss for metric learning and the relevant NormLinearClsHead.

Updating documents:

1. Update our OpenMixup tech report on arXiv, which provides more technical details and benchmark results.

Fixing bugs:

1. Fix raising error notification of torch.fft for PyTorch 1.6 or lower versions in backbones and heads.
2. Fix README.md (new icons, fixing typos) and support pytest in tests.

[Lupin1998]

OpenMixup V0.2.6 is now released. We will fix reported bugs and update documents of V0.2.6 if new issues are raised.