Support upstream yolov5 v6 release

[zhiqwang]

Following the upstream YOLOv5 release v6.0 ultralytics/yolov5#5141 . For release v6.0, use the snippet below

from yolort.models import YOLOv5

# 'yolov5s.pt' is downloaded from https://github.com/ultralytics/yolov5/releases/download/v6.0/yolov5s.pt
ckpt_path_from_ultralytics = "yolov5s.pt"
model = YOLOv5.load_from_yolov5(ckpt_path_from_ultralytics, version="r6.0")

model.eval()
img_path = "test/assets/bus.jpg"
predictions = model.predict(img_path)

Whereas for release v4.0 and v5.0, just change the version to version="r4.0" like below

# 'yolov5s.pt' is downloaded from https://github.com/ultralytics/yolov5/releases/download/v5.0/yolov5s.pt
# Or from https://github.com/ultralytics/yolov5/releases/download/v4.0/yolov5s.pt
ckpt_path_from_ultralytics = "yolov5s.pt"
model = YOLOv5.load_from_yolov5(ckpt_path_from_ultralytics, version="r4.0")

model.eval()
img_path = "test/assets/bus.jpg"
predictions = model.predict(img_path)

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Codecov Report

Merging #194 (8af0cb5) into master (d1f02f6) will not change coverage.
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@@           Coverage Diff           @@
##           master     #194   +/-   ##
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  Coverage   96.49%   96.49%           
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  Files          10       10           
  Lines         599      599           
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  Hits          578      578           
  Misses         21       21           

Flag	Coverage Δ
unittests	96.49% <ø> (ø)

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