[BugFix] Fix examples (#1173)

Author: vmoens

torchrl/objectives/dqn.py

@@ -56,7 +56,7 @@ class DQNLoss(LossModule):
 
     """
 
-    default_value_estimator = ValueEstimators.TDLambda
+    default_value_estimator = ValueEstimators.TD0
 
     def __init__(
         self,

torchrl/objectives/value/functional.py

@@ -58,16 +58,16 @@ def transpose_tensor(tensor):
                 or tensor.numel() <= 1
             ):
                 return tensor, False
-            if time_dim < 0:
-                timedim = tensor.ndim + time_dim
+            if time_dim >= 0:
+                timedim = time_dim - tensor.ndim
             else:
                 timedim = time_dim
-            if timedim < 0 or timedim >= tensor.ndim:
+            if timedim < -tensor.ndim or timedim >= 0:
                 raise RuntimeError(ERROR.format(tensor.shape, timedim))
             if tensor.ndim >= 2:
                 single_dim = False
                 tensor = tensor.transpose(timedim, -2)
-            elif tensor.ndim == 1 and timedim == 0:
+            elif tensor.ndim == 1 and timedim == -1:
                 single_dim = True
                 tensor = tensor.unsqueeze(-1)
             else: