add back sonnet dependency. If sonnet is not installed, fall back to simpleAgent that does not need sonnet.

examples/pybullet/gym/agents/actor_net.py

@@ -0,0 +1,21 @@
+"""An actor network."""
+import tensorflow as tf
+import sonnet as snt
+
+class ActorNetwork(snt.AbstractModule):
+  """An actor network as a sonnet Module."""
+
+  def __init__(self, layer_sizes, action_size, name='target_actor'):
+    super(ActorNetwork, self).__init__(name=name)
+    self._layer_sizes = layer_sizes
+    self._action_size = action_size
+
+  def _build(self, inputs):
+    state = inputs
+    for output_size in self._layer_sizes:
+      state = snt.Linear(output_size)(state)
+      state = tf.nn.relu(state)
+
+    action = tf.tanh(
+        snt.Linear(self._action_size, name='action')(state))
+    return action