HSwish operation specification (#1708)

* HSwish specification init

* Update docs/ops/activation/HSwish_4.md

Co-authored-by: Michał Karzyński <4430709+postrational@users.noreply.github.com>

* Update docs/ops/opset4.md

Co-authored-by: Michał Karzyński <4430709+postrational@users.noreply.github.com>

docs/ops/activation/HSwish_4.md

@@ -0,0 +1,50 @@
+## HSwish <a name="HSwish"></a> {#openvino_docs_ops_activation_HSwish_4}
+
+**Versioned name**: *HSwish-4*
+
+**Category**: *Activation*
+
+**Short description**: HSwish takes one input tensor and produces output tensor where the hard version of swish function is applied to the tensor elementwise.
+
+**Detailed description**: For each element from the input tensor calculates corresponding
+element in the output tensor with the following formula: 
+
+    \f[
+    HSwish(x) = x \frac{min(max(x + 3, 0), 6)}{6}
+    \f]
+
+The HSwish operation is introduced in the following [article](https://arxiv.org/pdf/1905.02244.pdf).
+
+**Attributes**: operation has no attributes.
+
+**Inputs**:
+
+*   **1**: Multidimensional input tensor of type *T*. **Required**.
+
+**Outputs**:
+
+*   **1**: The resulting tensor of the same shape and type as input tensor.
+
+**Types**
+
+* *T*: arbitrary supported floating point type.
+
+
+**Example**
+
+```xml
+<layer ... type="HSwish">
+    <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>
+```

docs/ops/opset4.md

@@ -64,6 +64,7 @@ declared in `namespace opset4`.
 * [GRUCell](sequence/GRUCell_3.md)
 * [GRUSequence](sequence/GRUSequence_4.md)
 * [HardSigmoid](activation/HardSigmoid_1.md)
+* [HSwish](activation/HSwish_4.md)
 * [Interpolate](image/Interpolate_4.md)
 * [Less](comparison/Less_1.md)
 * [LessEqual](comparison/LessEqual_1.md)