Bugfix: Provide default for MLLayerNormalizationOptions axes in steps

This adds an explicit algorithm step in layerNormalization() to
provide a default described only in prose for the axes option.

For webmachinelearning#211

index.bs

@@ -3820,6 +3820,7 @@ partial interface MLGraphBuilder {
     The <dfn method for=MLGraphBuilder>layerNormalization(|input|, |options|)</dfn> method steps are:
   </summary>
   <div class=algorithm-steps>
+    1. If |options|.{{MLLayerNormalizationOptions/axes}} does not [=map/exist=], set |options|.{{MLLayerNormalizationOptions/axes}} to the [=/list=] « 1, 2, 3 ».
     1. If the [=rank=] of |options|.{{MLLayerNormalizationOptions/scale}} is not equal to the [=list/size=] of |options|.{{MLLayerNormalizationOptions/axes}}, then [=exception/throw=] a "{{DataError}}" {{DOMException}}.
     1. If the [=rank=] of |options|.{{MLLayerNormalizationOptions/bias}} is not equal to the [=list/size=] of |options|.{{MLLayerNormalizationOptions/axes}}, then [=exception/throw=] a "{{DataError}}" {{DOMException}}.
     1. [=list/For each=] |index| in [=the range=] 0 to the [=list/size=] of |options|.{{MLLayerNormalizationOptions/axes}}, exclusive: