Add Visualizer for visualization

[zhiqwang]

We can use the Visualizer to display the detections

from torchvision.io import read_image
from yolort.models import yolov5n6
from yolort.utils import Visualizer

model = yolov5n6(pretrained=True, score_thresh=0.45)
model = model.eval()
img_path = "bus.jpg"
preds = model.predict(img_path)

metalabels_path = "coco.names"
image = read_image(img_path)
v = Visualizer(image, metalabels=metalabels_path)
v.draw_instance_predictions(preds[0])
v.imshow(scale=0.5)

Close #227

[codecov]

Codecov Report

Merging #341 (9386c16) into main (3a47430) will increase coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head 9386c16 differs from pull request most recent head 88b830a. Consider uploading reports for the commit 88b830a to get more accurate results

@@            Coverage Diff             @@
##             main     #341      +/-   ##
==========================================
+ Coverage   95.38%   95.46%   +0.07%     
==========================================
  Files          11       11              
  Lines         759      771      +12     
==========================================
+ Hits          724      736      +12     
  Misses         35       35              

Flag	Coverage Δ
unittests	95.46% <100.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
test/test_utils.py	78.18% <100.00%> (+2.67%)	⬆️

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