Update docstrings of MaskRCNN (#9083)

Co-authored-by: Nicolas Hug <[email protected]>

Author: hmk114

torchvision/models/detection/mask_rcnn.py

@@ -50,7 +50,7 @@ class MaskRCNN(FasterRCNN):
           ``0 <= x1 < x2 <= W`` and ``0 <= y1 < y2 <= H``.
         - labels (Int64Tensor[N]): the predicted labels for each image
         - scores (Tensor[N]): the scores or each prediction
-        - masks (UInt8Tensor[N, 1, H, W]): the predicted masks for each instance, in 0-1 range. In order to
+        - masks (FloatTensor[N, 1, H, W]): the predicted masks for each instance, in 0-1 range. In order to
           obtain the final segmentation masks, the soft masks can be thresholded, generally
           with a value of 0.5 (mask >= 0.5)