remove self.model_target and added a target q ensemble only without t…

…he need to copy the

 entire policy

lerobot/common/policies/tdmpc2/modeling_tdmpc2.py

@@ -39,7 +39,7 @@
 from lerobot.common.policies.normalize import Normalize, Unnormalize
 from lerobot.common.policies.tdmpc2.configuration_tdmpc2 import TDMPC2Config
 from lerobot.common.policies.utils import get_device_from_parameters, populate_queues
-from lerobot.common.policies.tdmpc2.tdmpc2_utils import NormedLinear, SimNorm, two_hot_inv
+from lerobot.common.policies.tdmpc2.tdmpc2_utils import NormedLinear, SimNorm, two_hot_inv, gaussian_logprob, squash
 
 
 class TDMPC2Policy(
@@ -84,9 +84,6 @@ def __init__(
             config = TDMPC2Config()
         self.config = config
         self.model = TDMPC2WorldModel(config)
-        self.model_target = deepcopy(self.model)
-        for param in self.model_target.parameters():
-            param.requires_grad = False
 
         if config.input_normalization_modes is not None:
             self.normalize_inputs = Normalize(
@@ -384,12 +381,12 @@ def forward(self, batch: dict[str, Tensor]) -> dict[str, Tensor | float]:
         # Compute various targets with stopgrad.
         with torch.no_grad():
             # Latent state consistency targets.
-            z_targets = self.model_target.encode(next_observations)
+            z_targets = self.model.encode(next_observations)
             # Compute the TD-target from a reward and the next observation
             pi = self.model.pi(z_targets)[0]
             td_targets = (
                 reward
-                + self.config.discount * self.model_target.Qs(z_targets, pi, return_type="min").squeeze()
+                + self.config.discount * self.model.Qs(z_targets, pi, return_type="min", target=True).squeeze()
             )
 
         # Compute losses.
@@ -450,10 +447,15 @@ def forward(self, batch: dict[str, Tensor]) -> dict[str, Tensor | float]:
         # Calculate the advantage weighted regression loss for π as detailed in FOWM 3.1.
         # We won't need these gradients again so detach.
         z_preds = z_preds.detach()
+        self.model.change_q_grad(mode=False)
         action_preds, _, log_pis, _ = self.model.pi(z_preds[:-1])
-        qs = self.model_target.Qs(z_preds[:-1], action_preds, return_type="avg")
-        self.scale.update(qs[0])
-        qs = self.scale(qs)
+
+        with torch.no_grad():
+            # avoid unnessecary computation of the gradients during policy optimization
+            # TODO (michel-aractingi): the same logic should be extended when adding task embeddings 
+            qs = self.model.Qs(z_preds[:-1], action_preds, return_type="avg")
+            self.scale.update(qs[0])
+            qs = self.scale(qs)
 
         rho = torch.pow(self.config.rho, torch.arange(len(qs), device=qs.device)).unsqueeze(-1)
 
@@ -498,12 +500,8 @@ def forward(self, batch: dict[str, Tensor]) -> dict[str, Tensor | float]:
         return info
 
     def update(self):
-        """Update the target model's parameters with an EMA step."""
-        # Note a minor variation with respect to the original FOWM code. Here they do this based on an EMA
-        # update frequency parameter which is set to 2 (every 2 steps an update is done). To simplify the code
-        # we update every step and adjust the decay parameter `alpha` accordingly (0.99 -> 0.995)
-        update_ema_parameters(self.model_target, self.model, self.config.target_model_momentum)
-
+        """Update the target model's using polyak averaging."""
+        self.model.update_target_Q()
 
 class TDMPC2WorldModel(nn.Module):
     """Latent dynamics model used in TD-MPC2."""
@@ -586,6 +584,11 @@ def to(self, *args, **kwargs):
         self.log_std_dif = self.log_std_dif.to(*args, **kwargs)
         self.bins = self.bins.to(*args, **kwargs)
         return self
+
+    def train(self, mode):
+        super().train(mode)
+        self._target_Qs.train(False)
+        return self
 
     def encode(self, obs: dict[str, Tensor]) -> Tensor:
         """Encodes an observation into its latent representation."""
@@ -622,7 +625,7 @@ def latent_dynamics(self, z: Tensor, a: Tensor) -> Tensor:
         x = torch.cat([z, a], dim=-1)
         return self._dynamics(x)
 
-    def pi(self, z: Tensor, std: float = 0.0) -> Tensor:
+    def pi(self, z: Tensor) -> Tensor:
         """Samples an action from the learned policy.
 
         The policy can also have added (truncated) Gaussian noise injected for encouraging exploration when
@@ -668,6 +671,14 @@ def Qs(self, z: Tensor, a: Tensor, return_type: str = "min", target=False) -> Te
         Q1, Q2 = two_hot_inv(Q1, self.bins), two_hot_inv(Q2, self.bins)
         return torch.min(Q1, Q2) if return_type == "min" else (Q1 + Q2) / 2
 
+    def update_target_Q(self):
+        """
+        Soft-update target Q-networks using Polyak averaging.
+        """
+        with torch.no_grad():
+            for p, p_target in zip(self._Qs.parameters(), self._target_Qs.parameters()):
+                p_target.data.lerp_(p.data, self.config.target_model_momentum)
+
 
 class TDMPC2ObservationEncoder(nn.Module):
     """Encode image and/or state vector observations."""
@@ -777,23 +788,6 @@ def random_shifts_aug(x: Tensor, max_random_shift_ratio: float) -> Tensor:
     return F.grid_sample(x, grid, padding_mode="zeros", align_corners=False)
 
 
-def update_ema_parameters(ema_net: nn.Module, net: nn.Module, alpha: float):
-    """Update EMA parameters in place with ema_param <- alpha * ema_param + (1 - alpha) * param."""
-    for ema_module, module in zip(ema_net.modules(), net.modules(), strict=True):
-        for (n_p_ema, p_ema), (n_p, p) in zip(
-            ema_module.named_parameters(recurse=False), module.named_parameters(recurse=False), strict=True
-        ):
-            assert n_p_ema == n_p, "Parameter names don't match for EMA model update"
-            if isinstance(p, dict):
-                raise RuntimeError("Dict parameter not supported")
-            if isinstance(module, nn.modules.batchnorm._BatchNorm) or not p.requires_grad:
-                # Copy BatchNorm parameters, and non-trainable parameters directly.
-                p_ema.copy_(p.to(dtype=p_ema.dtype).data)
-            with torch.no_grad():
-                p_ema.mul_(alpha)
-                p_ema.add_(p.to(dtype=p_ema.dtype).data, alpha=1 - alpha)
-
-
 def flatten_forward_unflatten(fn: Callable[[Tensor], Tensor], image_tensor: Tensor) -> Tensor:
     """Helper to temporarily flatten extra dims at the start of the image tensor.