Add crowd-counting models and tutorials

[la-cruche]

Hi,

Creating this issue to track interest in bringing crowd-counting models and tutos to gluoncv
Examples of crowd counting research:

• Deep Spatial Regression Model for Image Crowd Counting https://arxiv.org/ftp/arxiv/papers/1710/1710.09757.pdf (Yao et al)
• CNN-based Density Estimation and Crowd Counting: A Survey https://arxiv.org/pdf/2003.12783v1.pdf (Gao et al)

[github-actions]

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