Helper functions for reconstruction, retrieval and metric calculation

1. Implement `do_reconstruction_and_retrievals` for betas given model,
   diffusion engine and images to perform retrieval on.

2. `evaluate_results` function to calculate `alexnet`, `clip`, `inception`,
   `pixcorr`, `ssim`, `retrieval` metrics given reconstructions, ground
truth images and clip voxels

Author: Anish Mahishi