[Feature] Composite.separates

ghstack-source-id: fbfc4308a81cd96ecc61723df8c0eb870c442def
Pull Request resolved: #2599

torchrl/data/tensor_specs.py

@@ -4339,6 +4339,33 @@ def pop(self, key: NestedKey, default: Any = NO_DEFAULT) -> Any:
             return default
         raise KeyError(f"{key} not found in composite spec.")
 
+    def separates(self, *keys: NestedKey, default: Any = None) -> Composite:
+        """Splits the composite spec by extracting specified keys and their associated values into a new composite spec.
+
+        This method iterates over the provided keys, removes them from the current composite spec, and adds them to a new
+        composite spec. If a key is not found, the specified default value is used. The new composite spec is returned.
+
+        Args:
+            *keys (NestedKey):
+                One or more keys to be extracted from the composite spec. Each key can be a single key or a nested key.
+            default (Any, optional):
+                The value to use if a specified key is not found in the composite spec. Defaults to `None`.
+
+        Returns:
+            Composite: A new composite spec containing the extracted keys and their associated values.
+
+        Note:
+            If none of the specified keys are found, the method returns `None`.
+        """
+        out = None
+        for key in keys:
+            result = self.pop(key, default=default)
+            if result is not None:
+                if out is None:
+                    out = Composite(batch_size=self.batch_size, device=self.device)
+                out[key] = result
+        return out
+
     def set(self, name, spec):
         if self.locked:
             raise RuntimeError("Cannot modify a locked Composite.")