title: MTCNN does not use GPU on first detection but does on following detections
body: <p>I have tensorflow 2.0 -gpu installed. I am doing face detection using MTCNN. On the first call to detect the face it takes 3.86 seconds. On the next call it takes only .049 seconds. I suspect it is not using the GPU on the first call but it does on the second call. I know MTCNN does  import tensorflow but I do not understand why the GPU is not used on the first call. Code is below.</p>
  <pre><code>import time
  from mtcnn import MTCNN
  import cv2
  #********first run of image detection - note resulting process time- think not using gpu
  detector = MTCNN()
  img_file=r'c:\Temp\people\storage\1.jpg'   
  img = cv2.imread(img_file, cv2.COLOR_BGR2RGB)        
  start=time.time()
  detector.detect_faces(img)
  stop=time.time()
  duration = stop-start
  print(duration)
  # rerun image detection on the same image - note duration much less must be using gpu
  start=time.time()
  detector.detect_faces(img)
  stop=time.time()
  duration = stop-start
  print(duration)
  Using TensorFlow backend.
  3.8625590801239014
  0.049181222915649414
  </code></pre>