[BugFix] Fix shape mismatch in _set_index_in_td with trailing dims of 1

test/test_libs.py

@@ -64,6 +64,7 @@
     OneHot,
     ReplayBuffer,
     ReplayBufferEnsemble,
+    TensorDictReplayBuffer,
     Unbounded,
     UnboundedDiscrete,
 )
@@ -5128,6 +5129,34 @@ def test_collector(self, task, parallel):
         for _ in collector:
             break
 
+    def test_single_agent_group_replay_buffer(self):
+        """Regression test for gh#3515 - shape mismatch with single-agent group."""
+        env = PettingZooEnv(
+            task="simple_v3",
+            parallel=True,
+            seed=0,
+            use_mask=False,
+        )
+        group = list(env.group_map.keys())[0]
+        assert len(env.group_map[group]) == 1
+
+        rollout = env.rollout(10)
+        T = rollout.shape[0]
+        n_agents = 1
+
+        # Reshape to (1, T, n_agents) to reproduce the scenario from gh#3515
+        # where a replay buffer Transform reshapes collector output to
+        # (n_envs, traj_len, n_agents). When n_agents=1 the trailing dim of 1
+        # caused _set_index_in_td to match the wrong number of batch dims.
+        td = rollout.unsqueeze(0).unsqueeze(-1)
+        assert td.shape == torch.Size([1, T, n_agents])
+
+        rb = TensorDictReplayBuffer(
+            storage=LazyTensorStorage(10_000, ndim=3),
+            batch_size=4,
+        )
+        rb.extend(td)
+
 
 @pytest.mark.skipif(not _has_robohive, reason="RoboHive not found")
 class TestRoboHive:

torchrl/data/replay_buffers/replay_buffers.py

@@ -1657,7 +1657,7 @@ def _set_index_in_td(self, tensordict, index):
         if _is_int(index):
             index = torch.as_tensor(index, device=tensordict.device)
         elif index.ndim == 2 and index.shape[:1] != tensordict.shape[:1]:
-            for dim in range(2, tensordict.ndim + 1):
+            for dim in range(tensordict.ndim, 1, -1):
                 if index.shape[:1].numel() == tensordict.shape[:dim].numel():
                     # if index has 2 dims and is in a non-zero format
                     index = index.unflatten(0, tensordict.shape[:dim])

torchrl/envs/transforms/transforms.py

@@ -7920,8 +7920,12 @@ def _propagate_to_nested_keys(self, next_tensordict: TensorDictBase) -> None:
             for parent_key in self.truncated_keys:
                 parent_truncated = next_tensordict.get(parent_key, None)
                 if parent_truncated is not None:
-                    # Expand parent truncated to match nested shape and apply OR
-                    expanded = parent_truncated.expand_as(nested_truncated)
+                    # Insert extra dims (e.g. agent dims) so the parent is
+                    # broadcastable to the nested agent-level shape.
+                    parent_val = parent_truncated
+                    while parent_val.ndim < nested_truncated.ndim:
+                        parent_val = parent_val.unsqueeze(-2)
+                    expanded = parent_val.expand_as(nested_truncated)
                     next_tensordict.set(nested_key, nested_truncated | expanded)
                     break
 
@@ -7937,8 +7941,10 @@ def _propagate_to_nested_keys(self, next_tensordict: TensorDictBase) -> None:
                 for parent_key in self.done_keys:
                     parent_done = next_tensordict.get(parent_key, None)
                     if parent_done is not None:
-                        # Expand parent done to match nested shape and apply OR
-                        expanded = parent_done.expand_as(nested_done)
+                        parent_val = parent_done
+                        while parent_val.ndim < nested_done.ndim:
+                            parent_val = parent_val.unsqueeze(-2)
+                        expanded = parent_val.expand_as(nested_done)
                         next_tensordict.set(nested_key, nested_done | expanded)
                         break