[example] Raise warning message for ngp renderer (#7434)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

python/taichi/examples/rendering/taichi_ngp.py

@@ -13,15 +13,19 @@
 import time
 from typing import Tuple
 
-import cv2
 import numpy as np
-import wget
 from matplotlib import pyplot as plt
-from scipy.spatial.transform import Rotation as R
 
 import taichi as ti
 from taichi.math import uvec3, vec2, vec3
 
+try:
+    import cv2
+    import wget
+    from scipy.spatial.transform import Rotation as R
+except:
+    raise ImportError("The Taichi NGP renderer requires cv2, wget and scipy to be installed.")
+
 
 def depth2img(depth):
     depth = (depth - depth.min()) / (depth.max() - depth.min())