Load an image from file. Required keys are "img_prefix" and "img_info" (a dict that must contain the key "filename"). Added or updated keys are "filename", "img", "img_shape", "ori_shape" (same as `img_shape`), "pad_shape" (same as `img_shape`), "scale_factor" (1.0) and "img_norm_cf
source not stored for this graph (policy: none)