Debug Google colab notebook

[KwatMDPhD]

Caught by @dan-r95

https://colab.research.google.com/github/CSAILVision/semantic-segmentation-pytorch/blob/master/notebooks/DemoSegmenter.ipynb#scrollTo=-IvAYVzTXkcP

[KwatMDPhD]

Hi @dan-r95 ,

I look like what you are seeing is just a warning message, instead of an error. The cleaning process succeeds.
I updated the error message to include "skipping" so that people know the cleaning process just skipped the cell, instead of failed.

I hope this helps.

Best,
Kwat