Implement ValueDICE and some restructuring

src/util/nn_training.py

@@ -0,0 +1,9 @@
+def optimizer_factory(config, parameters):
+    optimizer_type = config['optimizer']
+    learning_rate = config['lr']
+    weight_decay = config['weight_decay']
+    if optimizer_type == 'adam':  
+        optimizer = torch.optim.Adam(parameters, lr=learning_rate, weight_decay=weight_decay)
+    else:
+        raise NotImplementedError
+    return optimizer