Thanks so much for your question! 
When defining a quantum function (a function that is passed to a QNode), non-differentiable parameters (such as inputs in this case) require a default value to be defined. This is how differentiable parameters of a quantum function are being tracked when creating a QNode.
In this specific case, the definition of the qnode function could be changed to def qnode(weights, inputs=None). This way weights is marked as differentiable and inputs as non-differentiable.
Further to this, there is an upcoming QNode using the new QuantumTape class. This is at the moment in experimental phase and compatibility with templates (such as AmplitudeEmbedding) is underway!
Hope this helps!