I think when you build a MaskRCNN object, the GeneralizedRCNNTransformation will be added as the first block of the model:
  (transform): GeneralizedRCNNTransform(                                              
      Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225])        
      Resize(min_size=(800,), max_size=1333, mode='bilinear')                                              
  ) 
  (backbone): Sequential(
    (0): Sequential(
      (0): ConvBNActivation(
        (0): Conv2d(3, 16, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1), bias=False)
        (1): FrozenBatchNorm2d(16)
        (2): Hardswish()
      )
      (1): InvertedResidual(
        (block): Sequential(
...

This is added by the MaskRCNN(), as the backbone before passing to MaskRCNN() did not have this transform block.