This PR adds a new training argument `--save-period` to save training checkpoints every `x` epochs. To save training every 50 epochs for example:
```
python train.py --save-period 50  # saves epoch50.pt, epoch100.pt, epoch150.pt, ... etc.
```

This saved checkpoints in addition to existing last.pt and best.pt checkpoints and does not affect their behavior. Default value is -1, i.e. disabled.

* Clip TTA Augmented Tails

Experimental TTA update.

* Update yolo.py

* Update yolo.py

* Update yolo.py

* Update yolo.py

* Fix E722, do not use bare 'except'

* Remove used codes

* Add FileNotFoundError in LoadImagesAndLabels

* Remove AssertionError

* Ignore LoadImagesAndLabels

* Ignore downloads.py

* Ignore torch_utils.py

* Ignore train.py

* Ignore datasets.py

* Enable utils/download.py

* Fixing exception in thop

* Remove unused code

* Fixing exception in LoadImagesAndLabels

* Fixing exception in exif_size

* Fixing exception in parse_model

* Ignore exceptions in requests

* Revert the exception as suggested

* Revert the exception as suggested

* added callbacks

* added back callback to main

* added save_dir to callback output

* merged in upstream

* removed ghost code

* added survey

* Update README.md
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* added callbacks

* added back callback to main

* added save_dir to callback output

* merged in upstream

* removed ghost code

* fixed parsing error for google temp links
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

Temporarily reverts https://github.com/ultralytics/yolov5/pull/4978 until torch 1.10 is released, which should resolve `urllib.error.HTTPError: HTTP Error 403: rate limit exceeded` errors generated by torch hub from GitHub actions runners.

* Update ci-testing.yml for Hub custom model tests

* Update ci-testing.yml

`is_valset_wandb_artifact` and `is_trainset_wandb_artifact` were referenced before assignment causing wandb to be unusable.

* added callbacks

* added back callback to main

* added save_dir to callback output

* merged in upstream

* removed ghost code

* added integrations section removed competition

* attempt column build

* attempt column build

* moved new to header

* removed center column

* utm

* Update README.md

* Update README.md

* updated logo sizes

* resized logos

* fixed links

* Update README.md

* Update README.md

* Update README.md
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* Fix relative `ROOT` Pytorch Hub custom model bug

* Update yolo.py

* fix isascii for python3.6

* update comment with python 3.7 note
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* Update relative `ROOT` logic

* python 3.9 Path().is_relative_to() removal

* Allow YOLOv5 execution from arbitrary `cwd`

* Fix str bugs

* Automatic Chinese fonts plotting

* Default PIL=False

`check_file()` is now limited to searching opt-in directories: /data, /models, /utils. This prevents large non-project directories like /.git and /venv from being searched, which may cause `check_file()` to slow significantly.

* Validate best.pt on train end

* 0.7 iou for COCO only

* pass callbacks

* active model.float() if not half

* print Validating best.pt...

* add newline

Add badge with link to join the new YOLOv5 Slack Forum!
https://join.slack.com/t/ultralytics/shared_invite/zt-w29ei8bp-jczz7QYUmDtgo6r6KcMIAg

* Fix `root` referenced before assignment

Fix for bug introduced by #4919 discovered on VOC autodownload:
```
python train.py --data VOC.yaml
```

* Cleanup

* Replace `os.system('unzip file.zip')` -> `ZipFile.extractall()`

* Cleanup

* Add arguments to TensorFlow NMS call

* Add regex substitution to reorder Identity_*

* Delete reorder in docstring

* Cleanup

* Cleanup2

* Removed `+ \` on string ends (not needed)
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* Avoid out-of-image class labels

* Update plots.py

* Cleanup
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>