CSPResNet-50 backbone — ResNet-50 with CSP-wrapped stages.

The CSPNet recipe applied to the ResNet-50 trunk (Wang et al., CVPRW 2020). Returns the final feature map only; pair with a classifier head via cspresnet_50_cls.

Reference: Wang et al., "CSPNet: A New Backbone that can Enhance Learning Capability of CNN", CVPRW 2020 (arXiv:1911.11929).