Style: Make grouped algorithm steps consistent

For operations defined in bulk - element-wise binary, unary, and
logical ops, plus pooling and reduction ops - make the styling
consistent:

- A single <div class=algorithm-steps> around all definitions
- A separate <div algorithm> around each op's steps.

This was already done for the element-wise ops so already the majority
style. Pooling ops now have just a single "Algorithm" watermark
(instead of 3), and reduction ops now have a watermark instead of
none.

For webmachinelearning#450

index.bs

@@ -4487,25 +4487,27 @@ partial interface MLGraphBuilder {
   <summary>
     The following pooling algorithms are supported.
   </summary>
-  <div algorithm class=algorithm-steps>
+  <div class=algorithm-steps>
+    <div algorithm>
     The <dfn method for=MLGraphBuilder>averagePool2d(|input|, |options|)</dfn> method steps are:
         1. Let |output| be the result of running the [=MLGraphBuilder/pooling-op | create pooling operation=] given "averagePool2d", |input| and |options|.
             1. If that [=exception/throws=] an error, then re-[=exception/throw=] the error.
         1. Return |output|.
-  </div>
+    </div>
 
-  <div algorithm class=algorithm-steps>
+    <div algorithm>
     The <dfn method for=MLGraphBuilder>l2Pool2d(|input|, |options|)</dfn> method steps are:
         1. Let |output| be the result of running the [=MLGraphBuilder/pooling-op | create pooling operation=] given "l2Pool2d", |input| and |options|.
             1. If that [=exception/throws=] an error, then re-[=exception/throw=] the error.
         1. Return |output|.
-  </div>
+    </div>
 
-  <div algorithm class=algorithm-steps>
+    <div algorithm>
     The <dfn method for=MLGraphBuilder>maxPool2d(|input|, |options|)</dfn> method steps are:
         1. Let |output| be the result of running the [=MLGraphBuilder/pooling-op | create pooling operation=] given "maxPool2d", |input| and |options|.
             1. If that [=exception/throws=] an error, then re-[=exception/throw=] the error.
         1. Return |output|.
+    </div>
   </div>
 </details>
 
@@ -4656,6 +4658,7 @@ partial interface MLGraphBuilder {
   <summary>
     The following reduce algorithms are supported.
   </summary>
+  <div class=algorithm-steps>
     <div algorithm>
     The <dfn method for=MLGraphBuilder>reduceL1(|input|, |options|)</dfn> method steps are:
         1. Let |output| be the result of running the [=MLGraphBuilder/reduce-op | create reduce operation=] given "reduceL1", |input| and |options|.
@@ -4725,6 +4728,7 @@ partial interface MLGraphBuilder {
             1. If that [=exception/throws=] an error, then re-[=exception/throw=] the error.
         1. Return |output|.
     </div>
+  </div>
 </details>
 
 ### relu ### {#api-mlgraphbuilder-relu-method}