Add support for ranger optimizers

Author: constantinpape

inferno/extensions/optimizers/__init__.py

@@ -1,2 +1,3 @@
 from .adam import Adam
-from .annealed_adam import AnnealedAdam
+from .annealed_adam import AnnealedAdam
+from .ranger import Ranger, RangerQH, RangerVA

inferno/extensions/optimizers/ranger.py

@@ -0,0 +1,8 @@
+# easy support for additional ranger optimizers from
+# https://github.com/lessw2020/Ranger-Deep-Learning-Optimizer
+try:
+    from ranger import Ranger, RangerVA, RangerQH
+except ImportError:
+    Ranger = None
+    RangerVA = None
+    RangerQH = None