[BugFix] Fix a bug in SliceSampler, indexes outside sampler lengths w…

…ere produced (#1874)

torchrl/data/replay_buffers/samplers.py

@@ -790,7 +790,7 @@ def _get_stop_and_length(self, storage, fallback=True):
                         raise RuntimeError(
                             "Could not get a tensordict out of the storage, which is required for SliceSampler to compute the trajectories."
                         )
-                vals = self._find_start_stop_traj(end=done.squeeze())[: len(storage)]
+                vals = self._find_start_stop_traj(end=done.squeeze()[: len(storage)])
                 if self.cache_values:
                     self._cache["stop-and-length"] = vals
                 return vals