Only allow the addition to Sequential objects of layers that are instances of Layer (keras-team#4184)

* Check that the added object is an instance of class Layer

* Update models.py

* Fix ValueError error message

Author: lgautier

keras/models.py

@@ -8,7 +8,7 @@
 from . import backend as K
 from .utils.io_utils import ask_to_proceed_with_overwrite
 from .engine.training import Model
-from .engine.topology import get_source_inputs, Node
+from .engine.topology import get_source_inputs, Node, Layer
 from .optimizers import optimizer_from_config
 from .legacy.models import Graph
 
@@ -260,6 +260,10 @@ def add(self, layer):
         # Arguments
             layer: layer instance.
         '''
+        if not isinstance(layer, Layer):
+            raise ValueError('The added layer must be '
+                             'an instance of class Layer. '
+                             'Found: ' + str(layer))
         if not self.outputs:
             # first layer in model: check that it is an input layer
             if len(layer.inbound_nodes) == 0: