ultralytics/yolov5: v3.1 - Bug Fixes and Performance Improvements

This release aggregates various minor bug fixes and performance improvements since the main v3.0 release and incorporates PyTorch 1.7.0 compatibility updates. v3.1 models share weights with v3.0 models but contain minor module updates (inplace fields for nn.Hardswish() activations) for native PyTorch 1.7.0 compatibility. Breaking Changes 'GIoU' hyperparameter has been renamed to 'box' to better reflect a criteria-agnostic regression loss term (https://github.com/ultralytics/yolov5/pull/1120) Bug Fixes PyTorch 1.7 compatibility update. torch>=1.6.0 required, torch>=1.7.0 recommended (https://github.com/ultralytics/yolov5/pull/1233) GhostConv module bug fix (https://github.com/ultralytics/yolov5/pull/1176)…