Added Custom Initialisation for MXNet Heads (#86)

* Added NormalizedRSSInitializer, using same method as TensorFlow backend, but changed name since ‘columns’ have different meaning in dense layer weight matrix in MXNet.

* Added unit test for NormalizedRSSInitializer.

rl_coach/tests/architectures/mxnet_components/heads/test_head.py

@@ -0,0 +1,25 @@
+import mxnet as mx
+import numpy as np
+import os
+import pytest
+import sys
+sys.path.append(os.path.dirname(os.path.dirname(os.path.dirname(__file__))))
+
+
+from rl_coach.architectures.mxnet_components.heads.head import NormalizedRSSInitializer
+
+
+@pytest.mark.unit_test
+def test_normalized_rss_initializer():
+    target_rss = 0.5
+    units = 10
+    dense = mx.gluon.nn.Dense(units=units, weight_initializer=NormalizedRSSInitializer(target_rss))
+    dense.initialize()
+
+    input_data = mx.random.uniform(shape=(25, 5))
+    output_data = dense(input_data)
+
+    weights = dense.weight.data()
+    assert weights.shape == (10, 5)
+    rss = weights.square().sum(axis=1).sqrt()
+    np.testing.assert_almost_equal(rss.asnumpy(), np.tile(target_rss, units))