Hi @ashis.1ck, if you are using the Jetson Xavier NX Developer Kit with the SD card image, then it should have already come with CUDA installed. You can check this by looking for the files under /usr/local/cuda or running /usr/local/cuda/bin/nvcc --version
If you are using a production Jetson Xavier NX module that you flashed with SDK Manager, then SDK Manager should have installed CUDA after it flashed the device.
Regarding the Realsense D435i, perhaps others from the community can lend their experience with it. You might also want to check out this thread to get it working outside of ROS first: