Expose is_available API for torch.backends.mkldnn (pytorch#147432)

As the title stated.

Like torch.backends.mkl, torch.backends.openmp and so on, they all expose
is_available API for users.
Pull Request resolved: pytorch#147432
Approved by: https://github.com/albanD

Author: FFFrog

torch/backends/mkldnn/__init__.py

@@ -94,6 +94,9 @@ class MkldnnModule(PropModule):
     def __init__(self, m, name):
         super().__init__(m, name)
 
+    def is_available(self):
+        return is_available()
+
     enabled = ContextProp(torch._C._get_mkldnn_enabled, torch._C._set_mkldnn_enabled)
     deterministic = ContextProp(
         torch._C._get_mkldnn_deterministic, torch._C._set_mkldnn_deterministic