add and improve comments

src/transformers/modeling_utils.py

@@ -732,6 +732,7 @@ class PreTrainedModel(nn.Module):
             outputs = self(**model_inputs)
             next_token_logits = outputs[0][:, -1, :]
 
+            # if model has past, then set the past parameter to speed up decoding
             if self._has_past(outputs):
                 past = outputs[1]
 
@@ -819,6 +820,7 @@ class PreTrainedModel(nn.Module):
             outputs = self(**model_inputs)  # (batch_size * num_beams, cur_len, vocab_size)
             scores = outputs[0][:, -1, :]  # (batch_size * num_beams, vocab_size)
 
+            # if model has past, then set the past parameter to speed up decoding
             if self._has_past(outputs):
                 past = outputs[1]