Add pydot package (#956)

This is used by keras plot_model() function.

Included a test to prevent regression.
Improved the existing tf test with extra assertions.

The issue was raised by a user here: https://www.kaggle.com/c/jane-street-market-prediction/discussion/214494#1184233

Dockerfile

@@ -80,6 +80,7 @@ RUN apt-get install -y libfreetype6-dev && \
     pip install xgboost && \
     # Pinned to match GPU version. Update version together.
     pip install lightgbm==3.1.1 && \
+    pip install pydot && \
     pip install keras && \
     pip install keras-tuner && \
     pip install flake8 && \

tests/test_tensorflow.py

@@ -1,4 +1,5 @@
 import unittest
+import os.path
 
 import numpy as np
 import tensorflow as tf
@@ -37,8 +38,13 @@ def test_tf_keras(self):
             metrics=['accuracy'])
 
         model.fit(x_train, y_train, epochs=1)
-        model.evaluate(x_test, y_test)
-
+
+        result = model.evaluate(x_test, y_test)
+        self.assertEqual(2, len(result))
+
+        # exercices pydot path.
+        tf.keras.utils.plot_model(model, to_file="tf_plot_model.png")
+        self.assertTrue(os.path.isfile("tf_plot_model.png"))          
 
     def test_lstm(self):
         x_train = np.random.random((100, 28, 28))
@@ -58,7 +64,8 @@ def test_lstm(self):
             metrics=['accuracy'])
 
         model.fit(x_train, y_train, epochs=1)
-        model.evaluate(x_test, y_test)
+        result = model.evaluate(x_test, y_test)
+        self.assertEqual(2, len(result))
 
     @gpu_test
     def test_gpu(self):