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    <li>Design Doc: Gradient Operators Registration</li>
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  <div class="section" id="design-doc-gradient-operators-registration">
<span id="design-doc-gradient-operators-registration"></span><h1>Design Doc: Gradient Operators Registration<a class="headerlink" href="#design-doc-gradient-operators-registration" title="Permalink to this headline"></a></h1>
<div class="section" id="the-problem-posed">
<span id="the-problem-posed"></span><h2>The Problem Posed<a class="headerlink" href="#the-problem-posed" title="Permalink to this headline"></a></h2>
<p>In our current operator registration mechanism, for each operator, the programmer should register a <em>gradient operator creator</em> function, which takes a C++ operator instance, and returns the corresponding gradient instance.</p>
<p>However, as we decided to separate the <em>compilation</em> and <em>execution</em> of DL models, we need to reshape the creator to take a protobuf <code class="docutils literal"><span class="pre">OpDesc</span></code> message, and returns a corresponding message.</p>
<p>More than that, the new registration mechanism need to support the fact that an operators&#8217; gradient computation might be a composition of operators.</p>
</div>
<div class="section" id="current-implementation">
<span id="current-implementation"></span><h2>Current Implementation<a class="headerlink" href="#current-implementation" title="Permalink to this headline"></a></h2>
<p>OpInfos store in a association map which key is the operator type. The <code class="docutils literal"><span class="pre">grad_op_type</span></code> indicate associated gradient operator type. Operator can create gradient operator by <code class="docutils literal"><span class="pre">OpInfo::creator_</span></code> of gradient. The pseudo code is</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">OpInfo</span> <span class="p">{</span>
  <span class="n">std</span><span class="o">::</span><span class="n">function</span><span class="o">&lt;</span><span class="n">OperatorBase</span><span class="o">*</span><span class="p">(...)</span><span class="o">&gt;</span> <span class="n">creator_</span><span class="p">;</span>
  <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">grad_op_type_</span><span class="p">;</span>
  <span class="p">...</span>
<span class="p">};</span>

<span class="n">map</span><span class="o">&lt;</span><span class="n">string</span><span class="p">,</span> <span class="n">OpInfo</span><span class="o">&gt;</span> <span class="n">OpInfoMap</span><span class="p">;</span>

<span class="n">OperatorBase</span><span class="o">*</span> <span class="nf">CreateGradientOperator</span><span class="p">(</span><span class="k">const</span> <span class="n">OperatorBase</span><span class="o">&amp;</span> <span class="n">op</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">return</span> <span class="n">OpInfoMap</span><span class="p">.</span><span class="n">at</span><span class="p">(</span><span class="n">op</span><span class="p">.</span><span class="n">Type</span><span class="p">()).</span><span class="n">creator_</span><span class="p">(...);</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="proposed-solution">
<span id="proposed-solution"></span><h2>Proposed Solution<a class="headerlink" href="#proposed-solution" title="Permalink to this headline"></a></h2>
<p>The mapping relationship between an operator and its gradient operators is a function. The interface of that function is:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="c1">// (OpDesc) --&gt; vector&lt;OpDesc&gt;</span>
<span class="k">using</span> <span class="n">GradOpDescMaker</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">function</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">OpDesc</span><span class="o">&gt;</span><span class="p">(</span><span class="k">const</span> <span class="n">OpDesc</span><span class="o">&amp;</span><span class="p">)</span><span class="o">&gt;</span><span class="p">;</span>
</pre></div>
</div>
<p>The function take a <code class="docutils literal"><span class="pre">OpDesc</span></code> of the forward operator and return one or many gradient operator descriptions.</p>
<p>The <code class="docutils literal"><span class="pre">GradOpDescMaker</span></code> will be registered in <code class="docutils literal"><span class="pre">OpInfo</span></code>, to replace <code class="docutils literal"><span class="pre">grad_op_type_</span></code> field. The <code class="docutils literal"><span class="pre">OpInfo</span></code> should be</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">OpInfo</span> <span class="p">{</span>
  <span class="n">GradOpDescMaker</span> <span class="n">grad_op_maker_</span><span class="p">;</span>
  <span class="p">...</span>
<span class="p">};</span>
</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">grad_op_maker_</span></code> is <code class="docutils literal"><span class="pre">nullptr</span></code> if the operator does not have associated gradient operators.</p>
<p>We should chagne register macros at the same time. In the current solution, there is no difference between forwarding operators and backward operators. So <code class="docutils literal"><span class="pre">REGISTER_OP</span></code> just register one operator. If the <code class="docutils literal"><span class="pre">REGISTER_OPERATOR</span></code> contains <code class="docutils literal"><span class="pre">OpProtoAndCheckerMaker</span></code> and <code class="docutils literal"><span class="pre">GradOpDescMaker</span></code>, we just list them in the same macro. It can be done by a macro contains <code class="docutils literal"><span class="pre">__VA_ARGS__</span></code>.</p>
<p>The user interface should be</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">vector</span><span class="o">&lt;</span><span class="n">OpDesc</span><span class="o">&gt;</span> <span class="n">MinusOpGradMaker</span><span class="p">(</span><span class="n">OpDesc</span><span class="p">)</span> <span class="p">{...}</span>
<span class="n">REGISTER_OPERATOR</span><span class="p">(</span><span class="n">minus</span><span class="p">,</span> <span class="n">MinusOp</span><span class="p">,</span> <span class="n">MinusOpProtoAndCheckerMaker</span><span class="p">,</span> <span class="n">SumOpGradMaker</span><span class="p">);</span>
<span class="c1">// Developers can still manually implement gradient operator.</span>
<span class="n">REGISTER_OPERATOR</span><span class="p">(</span><span class="n">minus_grad</span><span class="p">,</span> <span class="n">MinusGradOp</span><span class="p">);</span>
</pre></div>
</div>
<p>The interface of current <code class="docutils literal"><span class="pre">REGISTER_OP</span></code> macro could not be changed. In <code class="docutils literal"><span class="pre">REGISTER_OP</span></code>, it will invoke <code class="docutils literal"><span class="pre">REGISTER_OPERATOR</span></code> two times and generate GradOpDescMaker inside.</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">REGISTER_OP</span><span class="p">(</span><span class="n">minus</span><span class="p">,</span> <span class="n">MinusOp</span><span class="p">,</span> <span class="n">MinusOpProtoAndCheckerMaker</span><span class="p">,</span> <span class="n">minus_grad</span><span class="p">,</span> <span class="n">MinusGradOp</span><span class="p">);</span>
</pre></div>
</div>
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