!13398 complement the docs of CTCLoss operator.

From: @wangshuide2020
Reviewed-by: @liangchenghui,@wuxuejian
Signed-off-by: @liangchenghui

mindspore/ops/operations/nn_ops.py

@@ -7185,6 +7185,9 @@ class CTCLoss(PrimitiveWithInfer):
     """
     Calculates the CTC (Connectionist Temporal Classification) loss and the gradient.
 
+    The CTC algorithm is proposed in `Connectionist Temporal Classification: Labeling Unsegmented Sequence Data with
+    Recurrent Neural Networks <http://www.cs.toronto.edu/~graves/icml_2006.pdf>`_.
+
     Args:
         preprocess_collapse_repeated (bool): If true, repeated labels will be collapsed prior to the CTC calculation.
                                              Default: False.