Fix import error in Lightning (#5155)

nni/nas/evaluator/pytorch/__init__.py

 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT license.
 
-from .lightning import *
+import warnings
+
+try:
+    from .lightning import *
+except ImportError:
+    warnings.warn("PyTorch-Lightning must be installed to use PyTorch in NAS. "
+                  "If you are not using PyTorch, please `nni.set_default_framework('none')`")
+    raise