Parallelize detensorizing context over batch (#3729)

* Batch context.tolist()

* Cleanup

* Suggestion

parlai/core/torch_generator_agent.py

@@ -1005,6 +1005,19 @@ def _get_context(self, batch, batch_idx):
             ctxt = batch.full_text_vec[batch_idx]
         return ctxt
 
+    def _get_batch_context(self, batch):
+        """
+        Version of TGA._get_context() that operates on full batches for speed.
+        """
+        if self.beam_context_block_ngram <= 0:
+            # We aren't context blocking, return empty tensor of the correct size
+            return torch.zeros(batch.batchsize, 0, dtype=torch.long)
+
+        ctxt = batch.text_vec
+        if self.beam_block_full_context:
+            ctxt = batch.full_text_vec
+        return ctxt
+
     def _get_initial_decoder_input(
         self, bsz: int, beam_size: int, dev: torch.device
     ) -> torch.LongTensor:
@@ -1092,9 +1105,10 @@ def _generate(
         bsz = batch.batchsize
         if batch.text_vec is not None:
             batchsize = batch.batchsize
+            batch_context_list = self._get_batch_context(batch).tolist()
             beams = [
                 self._treesearch_factory(dev)
-                .set_context(self._get_context(batch, batch_idx))
+                .set_batch_context(batch_context_list, batch_idx)
                 .set_block_list(self.beam_block_list)
                 for batch_idx in range(batchsize)
             ]
@@ -1284,6 +1298,22 @@ def set_context(self: TSType, context: torch.LongTensor) -> TSType:
         self.context = context.tolist()
         return self
 
+    def set_batch_context(
+        self: TSType, batch_context_list: List[List[int]], batch_idx: int
+    ) -> TSType:
+        """
+        Version of .set_context() that operates on a single element of a batch.
+
+        Set the internal context representation and return self.
+
+        :param batch_context_list:
+            a list of lists, each one containing the context for one member of the batch
+        :param batch_idx:
+            index of the batch
+        """
+        self.context = batch_context_list[batch_idx]
+        return self
+
     def set_block_list(self: TSType, block_list: Optional[SearchBlocklist]) -> TSType:
         self.block_list = block_list
         return self