Fix - Cellpose2D notebook - Variables not defined

[antho214]

I am creating a pull request that solves some issues that I have been experiencing when using the Cellpose 2D notebook with the pretrained models.

Reproducibility

To reproduce the errors, run sections 1-2 and then move to section 5 to assess the quality of the pretrained models. I am using the pretrained model Cytoplasm3 with my own data.

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Problem
Some variables were not defined when jumping from sections 1-2 to section 5 to use the more recent pretrained models (e.g. Cytoplasm3). Without these modifications, this raised the following error when loading the Cytoplasm3 model.

NameError: name 'model_choice' is not defined

Solution
I have changed the variable name model_choice to QC_model as for the other models.

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Problem
Similarly, in section 5, some of the models (e.g. TissueNet) defined the variable channels which required segment_channel and nuclear_channel to be defined.
Solution
Since channels is not required in this cell and would be overwritten in section 5.2, I have removed the definition of channels.

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Problem
Lastly, some of the models (e.g. Cytoplasm3) did not include the default QC_model_folder and QC_model_name which raised an error when creating the output folder.

FileNotFoundError: [Errno 2] No such file or directory: ''

Solution
I have added the default QC_model_folder and QC_model_name to every model. This solves the issue.

Hopefully this helps!
Cheers!