Add lr_mult label to the network graph in draw_net.py (#6273)

draw_net.py refactoring and optional LR visualization

* refactoring `get_layer_label`

rewrote the function body to make it more streamlined.
does not affect inputs and outputs

* optionally visualize LR when drawing the network

adds an option to `python/draw_net.py` that allows to visualize information
about the learning rate multiplier (if relevant) when drawing the network's
graph.