Plotting convergence of alpha weights in One-Shot Optimizers

[jr2021]

Finalized Lukas' idea for heat map to visualize the convergence of alpha weights for One-Shot optimizers, via two new configuration parameters config.save_arch_weights and config.plot_arch_weights

Plotting was made to be extensible to larger search spaces by limiting the number of edges to 4, but this could also be parameterized if the user wants to be able to control this too.