Re-migrated diagnostics

[jerrymhuang]

WIP: remigrated the elements in the original diagnostics module from streamlined-backend to dev.

Currently include all existing features:

• plot_losses;
• plot_recovery;
• plot_sbc_ecdf;
• plot_sbc_historgram;
• plot_z_score_contraction;
• plot_distribution_2d.
• Inherited plots from plot_distribution_2d: plot_prior_2d, plot_posterior_2d, plot_latent_space_2d;
• plot_calibration_curves;
• plot_confusion_matrix;
• plot_mmd_hypothesis_test.

Still to come:

• Duplicate removal (through plot_utils);
• Quickstart notebook for demonstrating the use of remigrated modules.

[stefanradev93]

Hi Jerry, can you pls make sure that the linter is passing?