[Feature] Provide URLs of STDC, Segmenter and Twins pretrained models (#1357)

configs/_base_/models/segmenter_vit-b16_mask.py

configs/_base_/models/twins_pcpvt-s_fpn.py

configs/_base_/models/twins_pcpvt-s_upernet.py

configs/segmenter/README.md

...segmenter/segmenter_vit-l_mask_8x1_512x512_160k_ade20k.py

...segmenter/segmenter_vit-s_mask_8x1_512x512_160k_ade20k.py

...segmenter/segmenter_vit-t_mask_8x1_512x512_160k_ade20k.py

configs/stdc/README.md

configs/stdc/stdc1_in1k-pre_512x1024_80k_cityscapes.py

configs/stdc/stdc2_in1k-pre_512x1024_80k_cityscapes.py

configs/twins/README.md

...twins/twins_pcpvt-b_fpn_fpnhead_8x4_512x512_80k_ade20k.py

...s/twins/twins_pcpvt-b_uperhead_8x2_512x512_160k_ade20k.py

...twins/twins_pcpvt-l_fpn_fpnhead_8x4_512x512_80k_ade20k.py

...s/twins/twins_pcpvt-l_uperhead_8x2_512x512_160k_ade20k.py

...s/twins/twins_svt-b_fpn_fpnhead_8x4_512x512_80k_ade20k.py

...igs/twins/twins_svt-b_uperhead_8x2_512x512_160k_ade20k.py

...s/twins/twins_svt-l_fpn_fpnhead_8x4_512x512_80k_ade20k.py

...igs/twins/twins_svt-l_uperhead_8x2_512x512_160k_ade20k.py

...s/twins/twins_svt-s_fpn_fpnhead_8x4_512x512_80k_ade20k.py