Fix model summary

pytorch_lightning/utilities/model_summary.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 """Utilities related to model weights summary."""
+import contextlib
 import logging
 from collections import OrderedDict
 from typing import Any, Dict, List, Optional, Tuple, Union
@@ -263,8 +264,8 @@ def _forward_example_input(self) -> None:
         mode = model.training
         model.eval()
 
-        assert trainer is not None
-        with torch.no_grad(), trainer.precision_plugin.forward_context():
+        forward_context = contextlib.nullcontext() if trainer is None else trainer.precision_plugin.forward_context()
+        with torch.no_grad(), forward_context:
             # let the model hooks collect the input- and output shapes
             if isinstance(input_, (list, tuple)):
                 model(*input_)