@Nathane_Berrebi, looking at the error:
/usr/local/lib/python3.7/dist-packages/torchvision/models/detection/generalized_rcnn.py in forward(self, images, targets)
     55         """
     56         if self.training and targets is None:
---> 57             raise ValueError("In training mode, targets should be passed")
     58         if self.training:
     59             assert targets is not None

ValueError: In training mode, targets should be passed

If self.training is True, then targets cannot be None. We want targets to be None, so self.training must be set to false.  To do this, we must call model.eval() at the start of the validation section.