Training segmentation example notebook fixes

[raffeyC]

What

• Added missing import statements and parameters
• Fixed typos and default data directory paths
• Added code to crop the test data to fit the format used for training and validation

Note: Variable length inputs might have worked in the past, but does not seem to work now. Further checks required to ensure code behavior is as expected.

Why

• Changes were made in order to make the buggy code used for https://deepcell.readthedocs.io/en/master/notebooks/Training-Segmentation.html run properly
• Done as an introductory task to codebase and workflow