refine bilinear tensor product doc

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");
   }