Support loading model weight from ultralytics

[zhiqwang]

We introduce a new interface to load models from ultralytics/yolov5

from yolort.models import yolov5s

# the following 'yolov5s.pt' is downloaded from https://github.com/ultralytics/yolov5/releases/download/v4.0/yolov5s.pt
ckpt_path_from_ultralytics = 'yolov5s.pt'
model = yolov5s(score_thresh=0.25)
model.load_from_yolov5(ckpt_path_from_ultralytics)

model.eval()
img_path = 'test/assets/bus.jpg'
results = model.predict(img_path)

Resolves #141 .

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NOTE: This PR will introduce BC-breaking changes, we're moving the common utils to yolort.v5, such as

https://github.com/zhiqwang/yolov5-rt-stack/blob/845d1b2b8ef7805492c7364ebdd72952efd89b4a/yolort/models/path_aggregation_network.py#L7

https://github.com/zhiqwang/yolov5-rt-stack/blob/845d1b2b8ef7805492c7364ebdd72952efd89b4a/yolort/models/darknet.py#L8

[codecov]

Codecov Report

Merging #167 (348f053) into master (b9c6720) will decrease coverage by 0.09%.
The diff coverage is 95.74%.

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
- Coverage   96.25%   96.16%   -0.10%     
==========================================
  Files          11       10       -1     
  Lines         561      600      +39     
==========================================
+ Hits          540      577      +37     
- Misses         21       23       +2     

Flag	Coverage Δ
unittests	96.16% <95.74%> (-0.10%)	⬇️

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Impacted Files	Coverage Δ
test/test_v5.py	87.50% <87.50%> (ø)
test/test_utils.py	96.82% <95.65%> (+7.93%)	⬆️
test/test_data_pipeline.py	100.00% <100.00%> (ø)
test/test_models.py	95.37% <100.00%> (+0.49%)	⬆️
test/test_models_anchor_utils.py	100.00% <100.00%> (ø)
test/test_models_common.py	100.00% <100.00%> (ø)

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