Add guard around 'somewhat' newer torch RAdam / NAdam imports

Author: rwightman

timm/optim/__init__.py

@@ -21,8 +21,13 @@
 from .sgdp import SGDP
 from .sgdw import SGDW
 
-# bring torch optim into timm.optim namespace for consistency
-from torch.optim import Adadelta, Adagrad, Adamax, Adam, NAdam, RAdam, RMSprop, SGD
+# bring common torch.optim Optimizers into timm.optim namespace for consistency
+from torch.optim import Adadelta, Adagrad, Adamax, Adam, AdamW, RMSprop, SGD
+try:
+    # in case any very old torch versions being used
+    from torch.optim import NAdam, RAdam
+except ImportError:
+    pass
 
 from ._optim_factory import list_optimizers, get_optimizer_class, get_optimizer_info, OptimInfo, OptimizerRegistry, \
     create_optimizer_v2, create_optimizer, optimizer_kwargs