layers_to_use should be a dict, not a list

[Mayukhdeb]

instead of using:

layers_to_use = [model.layer1, model.layer2]

and then accessing them in the custom function as:

layer_outputs[0] and layer_outputs[1]

we can define layers_to_use as a dict which makes custom_func much more intuitive

layers_to_use = {
    'layer1': model.layer1, 
    'layer2': model.layer2
}

and then accessing them in the custom function as:

layer_outputs['layer2'] and layer_outputs['layer1']