Add better error message for model.summary() (#11222)
Add better error message for model.summary() (#11222)

* Add better error message for model.summary()

Added extra points in the error message to provide a better error message. For detailed discussion consider checking issue #11217 on keras repo. 
Old error message: "This model has never been called, thus its weights have not yet been created, so no summary can be displayed. Build the model first (e.g. by calling it on some test data)."
New error message: "This model has not yet been built. Build the model first by calling build() or calling fit() with some data. Or specify input_shape or batch_input_shape in the first layer for automatic build"

* Fix indentation problem causing build error in python 2.7

keras-team/keras,keras-team/keras