Fix keras layer naming (the char ":" is not a valid char for a name of keras layer)

[doronHarnoy]

This PR is fixing a naming problem that is causing the call of Keras layers to fail.

In TensorFlow.Keras the char : is not supported for name_scope hence using it as a name for layer will cause the computation of a layer to fail ( running the computation of a layer is creating a named scope with the layer's name )

Converting the unsupported char : to the char _ instead is suggested here to fix that problem

Supported chars for a named scope can be seen here