Fix for making unditioned generation work. Identical output as before.

examples/run_pplm.py

@@ -481,6 +481,7 @@ def generate_text_pplm(
     one_hot_bows_vectors = build_bows_one_hot_vectors(bow_indices)
 
     grad_norms = None
+    last = None
     unpert_discrim_loss = 0
     loss_in_time = []
     for i in trange(length, ascii=True):
@@ -491,6 +492,7 @@ def generate_text_pplm(
         # run model forward to obtain unperturbed
         if past is None and output_so_far is not None:
             last = output_so_far[:, -1:]
+            if output_so_far.shape[1] > 1:
                 _, past, _ = model(output_so_far[:, :-1])
 
         unpert_logits, unpert_past, unpert_all_hidden = model(output_so_far)
@@ -510,6 +512,7 @@ def generate_text_pplm(
             accumulated_hidden = unpert_last_hidden[:, :-1, :]
             accumulated_hidden = torch.sum(accumulated_hidden, dim=1)
 
+            if past is not None:
                 pert_past, _, grad_norms, loss_this_iter = perturb_past(
                     past,
                     model,
@@ -531,6 +534,8 @@ def generate_text_pplm(
                     gamma=gamma,
                 )
                 loss_in_time.append(loss_this_iter)
+            else:
+                pert_past = past
 
         pert_logits, past, pert_all_hidden = model(last, past=pert_past)
         pert_logits = pert_logits[:, -1, :] / temperature  # + SMALL_CONST