Bug fix: softmax()'s axis argument should EnforceRange (webmachinelea…

…rning#746)

All input integers have [EnforceRange] applied to prevent weirdness...
except softmax()'s axis argument. Already present in the Chromium
prototype implementation; it looks like this argument was just missed
in 6023741.

index.bs

@@ -5441,7 +5441,7 @@ the N-D input tensor along the given axis.
 <script type=idl>
 partial interface MLGraphBuilder {
   MLOperand softmax(MLOperand input,
-                    unsigned long axis,
+                    [EnforceRange] unsigned long axis,
                     optional MLOperatorOptions options = {});
 };
 </script>