Update check for sequential models (#6305)
Update check for sequential models (#6305)

* Update layer_utils.py

Model is not sequential if there is a "merge" layer somewhere in the graph. So if a layer has multiple input layers ("inbound_layers"), the whole model is no longer sequential...

* Explanation of changed condition

Added a comment to explain the check for sequentiality in a model:
A model is not sequential if it has multiple nodes or if a layer has multiple inbound_layers

keras-team/keras,keras-team/keras