[FEATURE] Add segformer as feature extractors ...

[jiarenyf]

https://github.com/NVlabs/SegFormer/blob/master/mmseg/models/backbones/mix_transformer.py#L374 https://drive.google.com/drive/folders/1b7bwrInTW4VLEm27YawHOAMSMikga2Ia

I think segformer is a good backbone / feature extractor. It provides pretrained weights so I think maybe it is good to introduce into timm.

[rwightman]

@jiarenyf it does lool like it could make an interesting backbone, BUT, the license is non-commercial (which is kinda lame given that they've clearly used timm based transformer code and they're using other apache licensed open source mmdetection / mmsegmentation.

Could ask them if they'll change the license? Sometimes they do. I occasionally add weights that might have an NC restriction, but never code.