Fix #3321: Update NCCL dep to 2.7 following NVIDIA update

Dockerfile.build.tmpl

@@ -64,7 +64,7 @@ ENV TF_CUDA_PATHS "/usr,/usr/local/cuda-10.1,/usr/lib/x86_64-linux-gnu/"
 ENV TF_CUDA_VERSION 10.1
 ENV TF_CUDNN_VERSION 7.6
 ENV TF_CUDA_COMPUTE_CAPABILITIES 6.0
-ENV TF_NCCL_VERSION 2.4
+ENV TF_NCCL_VERSION 2.7
 
 # Common Environment Setup
 ENV TF_BUILD_CONTAINER_TYPE GPU