Add metric learning model zoo and tutorial

[la-cruche]

Hi,

Could gluoncv include model zoo and tutorial sections for metric learning? (triplet, arcloss etc). There are MXNet-based resources here and there (eg Fehervari et al Unbiased Evaluation of Deep Metric Learning Algorithms https://arxiv.org/pdf/1911.12528.pdf.) and https://github.com/THUFutureLab/gluon-face but it would be great to have a production-ready SOTA in the gluoncv API.

[github-actions[bot]]

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