[BugFix] PPOs with composite distribution (#2791)

Co-authored-by: Louis Faury <louis.faury@helsing.ai>

Author: louisfaury

torchrl/objectives/ppo.py

@@ -527,12 +527,9 @@ def _log_weight(
             self.actor_network
         ) if self.functional else contextlib.nullcontext():
             dist = self.actor_network.get_dist(tensordict)
-        if isinstance(dist, CompositeDistribution):
-            is_composite = True
-        else:
-            is_composite = False
 
-        # current log_prob of actions
+        is_composite = isinstance(dist, CompositeDistribution)
+
         if is_composite:
             action = tensordict.select(
                 *(
@@ -562,25 +559,26 @@ def _log_weight(
         log_prob = dist.log_prob(action)
         if is_composite:
             with set_composite_lp_aggregate(False):
+                if log_prob.batch_size != adv_shape:
+                    log_prob.batch_size = adv_shape
                 if not is_tensor_collection(prev_log_prob):
-                    # this isn't great, in general multihead actions should have a composite log-prob too
+                    # this isn't great: in general, multi-head actions should have a composite log-prob too
                     warnings.warn(
                         "You are using a composite distribution, yet your log-probability is a tensor. "
                         "Make sure you have called tensordict.nn.set_composite_lp_aggregate(False).set() at "
                         "the beginning of your script to get a proper composite log-prob.",
                         category=UserWarning,
                     )
-                if log_prob.batch_size != adv_shape:
-                    log_prob.batch_size = adv_shape
-                if (
-                    is_composite
-                    and not is_tensor_collection(prev_log_prob)
-                    and is_tensor_collection(log_prob)
-                ):
-                    log_prob = _sum_td_features(log_prob)
-                    log_prob.view_as(prev_log_prob)
+
+                    if is_tensor_collection(log_prob):
+                        log_prob = _sum_td_features(log_prob)
+                        log_prob.view_as(prev_log_prob)
 
         log_weight = (log_prob - prev_log_prob).unsqueeze(-1)
+        if is_tensor_collection(log_weight):
+            log_weight = _sum_td_features(log_weight)
+            log_weight = log_weight.view(adv_shape).unsqueeze(-1)
+
         kl_approx = (prev_log_prob - log_prob).unsqueeze(-1)
         if is_tensor_collection(kl_approx):
             kl_approx = _sum_td_features(kl_approx)
@@ -691,9 +689,6 @@ def forward(self, tensordict: TensorDictBase) -> TensorDictBase:
         log_weight, dist, kl_approx = self._log_weight(
             tensordict, adv_shape=advantage.shape[:-1]
         )
-        if is_tensor_collection(log_weight):
-            log_weight = _sum_td_features(log_weight)
-            log_weight = log_weight.view(advantage.shape)
         neg_loss = log_weight.exp() * advantage
         td_out = TensorDict({"loss_objective": -neg_loss})
         td_out.set("kl_approx", kl_approx.detach().mean())  # for logging
@@ -987,8 +982,6 @@ def forward(self, tensordict: TensorDictBase) -> TensorDictBase:
             # to different, unrelated trajectories, which is not standard. Still, it can give an idea of the weights'
             # dispersion.
             lw = log_weight.squeeze()
-            if not isinstance(lw, torch.Tensor):
-                lw = _sum_td_features(lw)
             ess = (2 * lw.logsumexp(0) - (2 * lw).logsumexp(0)).exp()
             batch = log_weight.shape[0]
 
@@ -1000,8 +993,6 @@ def forward(self, tensordict: TensorDictBase) -> TensorDictBase:
         gain2 = ratio * advantage
 
         gain = torch.stack([gain1, gain2], -1).min(dim=-1).values
-        if is_tensor_collection(gain):
-            gain = _sum_td_features(gain)
         td_out = TensorDict({"loss_objective": -gain})
         td_out.set("clip_fraction", clip_fraction)
         td_out.set("kl_approx", kl_approx.detach().mean())  # for logging
@@ -1291,8 +1282,6 @@ def forward(self, tensordict: TensorDictBase) -> TensorDict:
             tensordict_copy, adv_shape=advantage.shape[:-1]
         )
         neg_loss = log_weight.exp() * advantage
-        if is_tensor_collection(neg_loss):
-            neg_loss = _sum_td_features(neg_loss)
 
         with self.actor_network_params.to_module(
             self.actor_network