title: Multiply keras loss functions by constant
body: <p>Is it possible to symbolically multiply a built-in keras loss function by a constant? For example if I want a linear combination of the losses of two outputs.</p>
  
  <p>I could write a custom loss function, but will it not get compiled since it's python code, not symbolic Keras? I'm looking for a way to do it in pure Keras (or TF)</p>