Merge pull request #5875 from peterzhang2029/refine_op_doc

Fix latex syntax in  bilinear tensor product operator comment.

paddle/operators/bilinear_tensor_product_op.cc

@@ -77,11 +77,19 @@ class BilinearTensorProductOpMaker : public framework::OpProtoAndCheckerMaker {
     AddOutput("Out", "The output of bilinear_tensor_product operator.");
     AddComment(R"DOC(
 Bilinear Tensor Product operator.
-Given input X and Y, a 3D tensor weight, and bias. Each column of the
-output is computed by one slice i = 1, . . . , k of the tensor:
-
-    M =  (X W_i) \cdot Y
-    Out_i = \sum_i {M_i} + Bias_i
+Given input X and Y, a 3D tensor Weight and a Bias. Each column of the
+Output is computed by one slice $i = 1, . . . , k$ of the tensor:
+
+$$
+M =  (X W_i) * Y \\
+Out_i = \sum_j {M_j} + Bias_i
+$$
+
+Where $W_i$ is the $i$-th slice of Input(Weight);
+      $M_j$ is the $j$-th column of $M$;
+      $Out_i$ is the $i$-th column of Output(Out);
+      $Bias_i$ is a column vector, each element of it is equal to
+        the $i$-th element of $Bias$;
 
 )DOC");
   }