Add segmentation model based on vit16 architecture.

[ekellbuch]

Include segmentation model based on vit16+ architecture (https://arxiv.org/abs/2105.05633) following internal implementation in https://github.com/google-research/scenic.

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[dustinvtran]

Looks great! Subject to some minor changes.

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[ekellbuch]

@googlebot I signed it!

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[ekellbuch]

@googlebot I fixed it.

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[dustinvtran]

Merged in fe95e35.