[Versioning] python 3.8 compatibility fix

ghstack-source-id: b59931a5b6d3062593228cb16fb026438c3c6105
Pull Request resolved: #1127

tensordict/utils.py

@@ -22,7 +22,6 @@
 from collections.abc import KeysView
 from contextlib import nullcontext
 from copy import copy
-from dataclasses import _FIELDS, GenericAlias
 from functools import wraps
 from importlib import import_module
 from numbers import Number
@@ -75,6 +74,18 @@
 if TYPE_CHECKING:
     from tensordict.tensordict import TensorDictBase
 
+try:
+    from dataclasses import _FIELDS, GenericAlias
+except ImportError:
+    # python < 3.9
+    from dataclasses import _FIELDS
+
+    class GenericAlias:
+        """Placeholder."""
+
+        ...
+
+
 try:
     try:
         from torch._C._functorch import (  # @manual=fbcode//caffe2:torch