[SSD/PyT] Update Dockerfile

Update Dockerfile to use PyTorch 21.07 as base

PyTorch/Detection/SSD/Dockerfile

@@ -1,4 +1,4 @@
-ARG FROM_IMAGE_NAME=nvcr.io/nvidia/pytorch:21.05-py3
+ARG FROM_IMAGE_NAME=nvcr.io/nvidia/pytorch:21.07-py3
 FROM ${FROM_IMAGE_NAME}
 
 # Set working directory