Ability to update learningRate in MLP (#160)

* Allow people to update the learning rate

* Test for learning rate setter

docs/machine-learning/neural-network/multilayer-perceptron-classifier.md

@@ -45,6 +45,12 @@ $mlp->partialTrain(
 
 ```
 
+You can update the learning rate between partialTrain runs:
+
+```
+$mlp->setLearningRate(0.1);
+```
+
 ## Predict
 
 To predict sample label use predict method. You can provide one sample or array of samples:

src/Phpml/NeuralNetwork/Network/MultilayerPerceptron.php

@@ -95,6 +95,12 @@ abstract class MultilayerPerceptron extends LayeredNetwork implements Estimator,
         }
     }
 
+    public function setLearningRate(float $learningRate): void
+    {
+        $this->learningRate = $learningRate;
+        $this->backpropagation->setLearningRate($this->learningRate);
+    }
+
     /**
      * @param mixed $target
      */

src/Phpml/NeuralNetwork/Training/Backpropagation.php

@@ -25,6 +25,11 @@ class Backpropagation
     private $prevSigmas = null;
 
     public function __construct(float $learningRate)
+    {
+        $this->setLearningRate($learningRate);
+    }
+
+    public function setLearningRate(float $learningRate): void
     {
         $this->learningRate = $learningRate;
     }

tests/Phpml/NeuralNetwork/Network/MultilayerPerceptronTest.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace tests\Phpml\NeuralNetwork\Network;
+
+use Phpml\NeuralNetwork\Network\MultilayerPerceptron;
+use PHPUnit\Framework\TestCase;
+
+class MultilayerPerceptronTest extends TestCase
+{
+    public function testLearningRateSetter(): void
+    {
+        $mlp = $this->getMockForAbstractClass(
+            MultilayerPerceptron::class,
+            [5, [3], [0, 1], 1000, null, 0.42]
+        );
+
+        $this->assertEquals(0.42, $this->readAttribute($mlp, 'learningRate'));
+        $backprop = $this->readAttribute($mlp, 'backpropagation');
+        $this->assertEquals(0.42, $this->readAttribute($backprop, 'learningRate'));
+
+        $mlp->setLearningRate(0.24);
+        $this->assertEquals(0.24, $this->readAttribute($mlp, 'learningRate'));
+        $backprop = $this->readAttribute($mlp, 'backpropagation');
+        $this->assertEquals(0.24, $this->readAttribute($backprop, 'learningRate'));
+    }
+}