Vizualization update

• Created a new python file 'models.py'.
• Moved the inference part of the 'grid_pred' function (located under viz/img.py) into a 'inference' function (located under utils/models.py).
• Solved issue when only 1 image given to 'grid_pred'. Solved in 'inference', lines 75-79.
• Modified the 'Unet.ipynb' with the new version of 'grid_pred'.