Sigmoid revise (#2942)

* remove sigmoid_backprop

* Update Sigmoid spec

* Update Sigmoid spec

docs/ops/activation/Sigmoid_1.md

@@ -6,7 +6,17 @@
 
 **Short description**: Sigmoid element-wise activation function.
 
-**Attributes**: operations has no attributes.
+**Detailed description**: [Reference](https://deepai.org/machine-learning-glossary-and-terms/sigmoid-function)
+
+**Attributes**: *Sigmoid* operation has no attributes.
+
+**Mathematical Formulation**
+
+   For each element from the input tensor calculates corresponding
+    element in the output tensor with the following formula:
+    \f[
+    sigmoid( x ) = \frac{1}{1+e^{-x}}
+    \f]
 
 **Inputs**:
 
@@ -16,10 +26,22 @@
 
 *   **1**: Result of Sigmoid function applied to the input tensor *x*. Floating point tensor with shape and type matching the input tensor. Required.
 
-**Mathematical Formulation**
+**Example**
 
-   For each element from the input tensor calculates corresponding
-    element in the output tensor with the following formula:
-    \f[
-    sigmoid( x ) = \frac{1}{1+e^{-x}}
-    \f]
+```xml
+<layer ... type="Sigmoid">
+    <input>
+        <port id="0">
+            <dim>256</dim>
+            <dim>56</dim>
+        </port>
+    </input>
+    <output>
+        <port id="1">
+            <dim>256</dim>
+            <dim>56</dim>
+        </port>
+    </output>
+</layer>
+
+```