Plotting heatmap and thumbnails for test PANDA slides

[harshita-s]

In this PR:

• The concept of slides is added to the Lightening Container that currently takes in input only the tiles dataset for PANDA.
• Slides of interest are selected from the test set using top/bottom attention tiles for TP/FN.
• Thumbnails are plotted for slides of interest.
• Attention heatmaps are plotted for the selected tiles in the bag for the slides of interest. The heatmaps are overlay on the slides.
• The thumbnails and attention heatmaps are saved as standard outputs in outputs/fig folder. The filenames contain slide identifier (slide ID), with _thumbnail and _heatmap suffixes, respectively.

[ant0nsc]

Looking good!
You can simplify setting up the callbacks. Also, I would find it prudent to add a small set of tests to ensure that your plotting really does what you think it does.

[vale-salvatelli]

Reviewed. Major comment: I think we can reduce the number of parameters we pass around by making get_slide_dataset a method implemented in each child class. Agreed with Anton that some tests would help