Define best_ahc_ext_lambd

[pedromxavier]

This fixes a corner case where condition in line

cim-optimizer/cim_optimizer/solve_Ising.py

Line 540 in 353e18f

if np.amin(test_batch[3])/rand_scaling < best_energy:

is not met and variable best_ahc_ext_lambd is not defined, causing an error upon the following print(...format(...)).