text: Title: TypeError: 'int' object is not subscriptable
  Body: I try to do inference on your pretrained model following: https://github.com/deepinsight/insightface/blob/master/examples/demo_analysis.py
  Here's my code:
  ```
  import insightface
  from cv2 import cv2
  
  app = insightface.app.FaceAnalysis()
  app.prepare(0, det_size=640)
  image = cv2.imread("my-picture.jpg")
  faces = app.get(image)
  assert len(faces)==6
  rimg = app.draw_on(image, faces)
  cv2.imwrite("output.jpg", rimg)
  ```
  
  Error: 
  ```
  Traceback (most recent call last):
    File "E:\Work work\Python\Work\atin\FaceRecog\main.py", line 7, in <module>
      faces = app.get(image)
    File "E:\Anaconda\envs\insightface\lib\site-packages\insightface\app\face_analysis.py", line 59, in get
      bboxes, kpss = self.det_model.detect(img,
    File "E:\Anaconda\envs\insightface\lib\site-packages\insightface\model_zoo\retinaface.py", line 212, in detect
      model_ratio = float(input_size[1]) / input_size[0]
  TypeError: 'int' object is not subscriptable
  find model: .insightface\models\buffalo_l\w600k_r50.onnx recognition ['None', 3, 112, 112] 127.5 127.5
  set det-size: 640
  ```
  