fix object_detection_tutorial.ipynb instance segmentation example error

[syiming]

Description

📝 Fix errors in object_detection_tutorial.ipynb instance segmentation

• error listed in: Object Detection API Demo: Instance Segmentation example doesn't work #8888

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Tests

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[harishsai]

I am still getting the same issue

[ucalyptus]

somewhat #8840

[ucalyptus]

check #8840 and #9034

[bartoszcholewa]

After applying those changes, can confirm that object_detection_tutorial is positively finishing to the end with images with masks

[ucalyptus]

@bartoszcholewa you should check #9034 I guess.