Make torch.backends.mkldnn usable without import

Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/32055

Differential Revision: D19373220

Pulled By: ezyang

fbshipit-source-id: 50ab3ff70fc893c81123419c4d3cf2e3e48a0a93
diff --git a/torch/__init__.py b/torch/__init__.py
index 8b24067..d6f0c60 100644
--- a/torch/__init__.py
+++ b/torch/__init__.py
@@ -339,6 +339,7 @@
 import torch.testing
 import torch.backends.cuda
 import torch.backends.mkl
+import torch.backends.mkldnn
 import torch.backends.openmp
 import torch.backends.quantized
 import torch.quantization