added reame for pretrained weights download

pretrained_weights/README.md

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+# Model Pretrained Weights
+
+# Downloading model pretrained weights
+
+Download checkpoints for each model from the following locations:
+
+* MViTv2: Download from https://dl.fbaipublicfiles.com/detectron2/MViTv2/cascade_mask_rcnn_mvitv2_b_in12k_3x/f309003202/model_final_be5168.pkl and rename the file as `mvitv2_b_cascade_mask_rcnn.pkl`. 
+
+* SAM: Download from https://dl.fbaipublicfiles.com/segment_anything/sam_vit_h_4b8939.pth.