YOLOv5 v4.0 Release (ultralytics#1837)

* Update C3 module

* Update C3 module

* Update C3 module

* Update C3 module

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* update datasets

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* update attempt_downlaod()

* merge

* merge

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* parameterize eps

* comments

* gs-multiple

* update

* max_nms implemented

* Create one_cycle() function

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* update study.png

* update study.png

* Update datasets.py

PyTorch 1.7.0 Compatibility Updates (ultralytics#1233)

* torch 1.7.0 compatibility updates

* add inference verification

Update readme to remove apex dependency (ultralytics#727)

* the value should be a list to unpacking

* filter the None labels

* update readme to remove apex dependency

Update README.md

Update README.md