CopyNet: replace in-place tensor operation with out-of-place equivale…

…nt (#2925)

* remove in-place operation

* oops, fixed

allennlp/models/encoder_decoders/copynet_seq2seq.py

@@ -627,7 +627,7 @@ def _gather_final_log_probs(self,
         source_token_ids = state["source_token_ids"]
 
         # shape: [(batch_size, *)]
-        modified_log_probs_list: List[torch.Tensor] = [generation_log_probs]
+        modified_log_probs_list: List[torch.Tensor] = []
         for i in range(trimmed_source_length):
             # shape: (group_size,)
             copy_log_probs_slice = copy_log_probs[:, i]
@@ -648,7 +648,9 @@ def _gather_final_log_probs(self,
             selected_generation_log_probs = generation_log_probs.gather(1, source_to_target_slice.unsqueeze(-1))
             combined_scores = util.logsumexp(
                     torch.cat((selected_generation_log_probs, copy_log_probs_to_add), dim=1))
-            generation_log_probs.scatter_(-1, source_to_target_slice.unsqueeze(-1), combined_scores.unsqueeze(-1))
+            generation_log_probs = generation_log_probs.scatter(-1,
+                                                                source_to_target_slice.unsqueeze(-1),
+                                                                combined_scores.unsqueeze(-1))
             # We have to combine copy scores for duplicate source tokens so that
             # we can find the overall most likely source token. So, if this is the first
             # occurence of this particular source token, we add the log_probs from all other
@@ -676,6 +678,7 @@ def _gather_final_log_probs(self,
             # shape: (group_size,)
             left_over_copy_log_probs = copy_log_probs_slice + (1.0 - copy_log_probs_to_add_mask + 1e-45).log()
             modified_log_probs_list.append(left_over_copy_log_probs.unsqueeze(-1))
+        modified_log_probs_list.insert(0, generation_log_probs)
 
         # shape: (group_size, target_vocab_size + trimmed_source_length)
         modified_log_probs = torch.cat(modified_log_probs_list, dim=-1)