Don't flatten when there are multiple continuous actions

ml-agents/mlagents/trainers/ppo/optimizer_torch.py

             torch.clamp(r_theta, 1.0 - decay_epsilon, 1.0 + decay_epsilon) * advantage
         )
         policy_loss = -1 * ModelUtils.masked_mean(
-            torch.min(p_opt_a, p_opt_b).flatten(), loss_masks
+            torch.min(p_opt_a, p_opt_b), loss_masks
         )
         return policy_loss