Track equivalent prompts in greedy_forward_reachability()

[amanb2000]

Magic_Words/magic_words/greedy_forward.py

Line 274 in c2a3617

print("Updating pool...")

Y_to_U[y]['equivalent_u'] = [[u_1], [u_2], ...] satisfy y = argmax P(y' | u_i + x_0) for all u_i.