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    <li>Error Clip</li>
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  <div class="section" id="error-clip">
<span id="error-clip"></span><h1>Error Clip<a class="headerlink" href="#error-clip" title="Permalink to this headline"></a></h1>
<div class="section" id="overview">
<span id="overview"></span><h2>Overview<a class="headerlink" href="#overview" title="Permalink to this headline"></a></h2>
<p>Error clip is widely used in model training to prevent gradient exploding. It takes some specific rules to adjust variables&#8217; gradients and prevent them from being too large. With it, values of a gradient will be checked before they are taken by the next <code class="docutils literal"><span class="pre">grad_op</span></code> and be shrunk if necessary.</p>
</div>
<div class="section" id="usage">
<span id="usage"></span><h2>Usage<a class="headerlink" href="#usage" title="Permalink to this headline"></a></h2>
<p>Users are allowed to assign different error clip methods or attributes to different <code class="docutils literal"><span class="pre">Variable</span></code>s. Users can specify it as a parameter of <code class="docutils literal"><span class="pre">Variable</span></code>&#8216;s constructor:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="n">var</span> <span class="o">=</span> <span class="n">framework</span><span class="o">.</span><span class="n">Variable</span><span class="p">(</span><span class="o">...</span><span class="p">,</span> <span class="n">error_clip</span><span class="o">=</span><span class="n">myErrorClip</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
</pre></div>
</div>
<p>The default value of <code class="docutils literal"><span class="pre">error_clip</span></code> is <code class="docutils literal"><span class="pre">None</span></code>, which means no error clip is employed. When it&#8217;s not <code class="docutils literal"><span class="pre">None</span></code>, it should take an object of <code class="docutils literal"><span class="pre">BaseErrorClipAttr</span></code>&#8216;s derived class. So far, <code class="docutils literal"><span class="pre">BaseErrorClipAttr</span></code> has only one derived class: <code class="docutils literal"><span class="pre">ErrorClipByValue</span></code>, whose constructor is:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="n">ErrorClipByValue</span><span class="p">(</span><span class="nb">max</span><span class="p">,</span> <span class="nb">min</span><span class="o">=</span><span class="bp">None</span><span class="p">)</span>
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">max</span></code> and <code class="docutils literal"><span class="pre">min</span></code> represent the maximal and minimal clip threshold respectively. In backward pass, all values of <code class="docutils literal"><span class="pre">var</span></code>&#8216;s gradient greater than <code class="docutils literal"><span class="pre">max</span></code> or less than <code class="docutils literal"><span class="pre">min</span></code> will be clipped to <code class="docutils literal"><span class="pre">max</span></code> and <code class="docutils literal"><span class="pre">min</span></code> respectively. When the <code class="docutils literal"><span class="pre">min</span></code> is None, the minimal threshold will be assigned with <code class="docutils literal"><span class="pre">-max</span></code> automatically.</p>
<p>So we can enable the error clip with threshold <code class="docutils literal"><span class="pre">[-5.0,</span> <span class="pre">5.0]</span></code> for variable <code class="docutils literal"><span class="pre">var</span></code> by:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="n">var</span> <span class="o">=</span> <span class="n">framework</span><span class="o">.</span><span class="n">Variable</span><span class="p">(</span><span class="o">...</span><span class="p">,</span> <span class="n">error_clip</span><span class="o">=</span><span class="n">ErrorClipByValue</span><span class="p">(</span><span class="nb">max</span><span class="o">=</span><span class="mf">5.0</span><span class="p">),</span> <span class="o">...</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="implementation">
<span id="implementation"></span><h2>Implementation<a class="headerlink" href="#implementation" title="Permalink to this headline"></a></h2>
<p>The <code class="docutils literal"><span class="pre">BaseErrorClipAttr</span></code> and its derived class <code class="docutils literal"><span class="pre">ErrorClipByValue</span></code> are defined in <em>clip.py</em>.</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">BaseErrorClipAttr</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
    <span class="k">def</span> <span class="nf">append_clip_op</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">block</span><span class="p">,</span> <span class="n">grad_name</span><span class="p">):</span>
        <span class="k">raise</span> <span class="ne">NotImplementedError</span><span class="p">()</span>


<span class="k">class</span> <span class="nc">ErrorClipByValue</span><span class="p">(</span><span class="n">BaseErrorClipAttr</span><span class="p">):</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">max</span><span class="p">,</span> <span class="nb">min</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
        <span class="nb">max</span> <span class="o">=</span> <span class="nb">float</span><span class="p">(</span><span class="nb">max</span><span class="p">)</span>
        <span class="k">if</span> <span class="nb">min</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
            <span class="nb">min</span> <span class="o">=</span> <span class="o">-</span><span class="nb">max</span>
        <span class="k">else</span><span class="p">:</span>
            <span class="nb">min</span> <span class="o">=</span> <span class="nb">float</span><span class="p">(</span><span class="nb">min</span><span class="p">)</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">max</span> <span class="o">=</span> <span class="nb">max</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">min</span> <span class="o">=</span> <span class="nb">min</span>

    <span class="k">def</span> <span class="nf">append_clip_op</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">block</span><span class="p">,</span> <span class="n">grad_name</span><span class="p">):</span>
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        <span class="n">clip_op_desc</span> <span class="o">=</span> <span class="n">block</span><span class="o">.</span><span class="n">desc</span><span class="o">.</span><span class="n">append_op</span><span class="p">()</span>
        <span class="n">clip_op_desc</span><span class="o">.</span><span class="n">set_type</span><span class="p">(</span><span class="s2">&quot;clip&quot;</span><span class="p">)</span>
        <span class="n">clip_op_desc</span><span class="o">.</span><span class="n">set_input</span><span class="p">(</span><span class="s2">&quot;X&quot;</span><span class="p">,</span> <span class="p">[</span><span class="n">grad_name</span><span class="p">])</span>
        <span class="n">clip_op_desc</span><span class="o">.</span><span class="n">set_output</span><span class="p">(</span><span class="s2">&quot;Out&quot;</span><span class="p">,</span> <span class="p">[</span><span class="n">grad_name</span><span class="p">])</span>
        <span class="n">clip_op_desc</span><span class="o">.</span><span class="n">set_attr</span><span class="p">(</span><span class="s2">&quot;min&quot;</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">min</span><span class="p">)</span>
        <span class="n">clip_op_desc</span><span class="o">.</span><span class="n">set_attr</span><span class="p">(</span><span class="s2">&quot;max&quot;</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">max</span><span class="p">)</span>
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</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">BaseErrorClipAttr</span></code> have one main member functions: <code class="docutils literal"><span class="pre">append_clip_op(self,</span> <span class="pre">block,</span> <span class="pre">grad_name)</span></code>.</p>
<p>This function is used to create a <code class="docutils literal"><span class="pre">clip_op</span></code> and append it to the end of given <code class="docutils literal"><span class="pre">block</span></code>. For different error clip algorithm require different <code class="docutils literal"><span class="pre">clip_op</span></code>, the function is defined as virtual in the base class. All derived classes must implement their own versions of this function.</p>
<p>These <code class="docutils literal"><span class="pre">clip_op</span></code>s should be inserted after <code class="docutils literal"><span class="pre">grad_op</span></code>s whose output gradients need to be clipped. It is equivalent to appending some <code class="docutils literal"><span class="pre">clip_op</span></code>s to the end of the target block every time a new <code class="docutils literal"><span class="pre">grad_op</span></code> is added.</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">op_desc</span> <span class="ow">in</span> <span class="n">grad_op_descs</span><span class="p">:</span>
        <span class="n">new_op_desc</span> <span class="o">=</span> <span class="n">target_block</span><span class="o">.</span><span class="n">desc</span><span class="o">.</span><span class="n">append_op</span><span class="p">()</span>
        <span class="n">new_op_desc</span><span class="o">.</span><span class="n">copy_from</span><span class="p">(</span><span class="n">op_desc</span><span class="p">)</span>
        <span class="n">callback</span><span class="p">(</span><span class="n">block</span><span class="o">=</span><span class="n">target_block</span><span class="p">,</span> <span class="n">context</span><span class="o">=</span><span class="n">grad_to_var</span><span class="p">)</span>
</pre></div>
</div>
<p>Here we employ a callback function to complete this kind of jobs. In <code class="docutils literal"><span class="pre">_append_backward_ops_</span></code> function, each time after a <code class="docutils literal"><span class="pre">grad_op</span></code> is added to the <code class="docutils literal"><span class="pre">target_block</span></code>, a callback function is invoked. The logic of <code class="docutils literal"><span class="pre">clip_op</span></code> appending can be implemented inside the callback function.</p>
<p>The callback function for <code class="docutils literal"><span class="pre">clip_op</span></code> appending is defined in <em>clip.py</em>:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">error_clip_callback</span><span class="p">(</span><span class="n">block</span><span class="p">,</span> <span class="n">context</span><span class="p">):</span>
    <span class="c1"># the context is a grad_to_var map</span>
    <span class="n">grad_to_var</span> <span class="o">=</span> <span class="n">context</span>
    <span class="n">op_desc</span> <span class="o">=</span> <span class="n">block</span><span class="o">.</span><span class="n">desc</span><span class="o">.</span><span class="n">op</span><span class="p">(</span><span class="n">block</span><span class="o">.</span><span class="n">desc</span><span class="o">.</span><span class="n">op_size</span><span class="p">()</span> <span class="o">-</span> <span class="mi">1</span><span class="p">)</span>
    <span class="k">for</span> <span class="n">grad_n</span> <span class="ow">in</span> <span class="nb">filter</span><span class="p">(</span><span class="k">lambda</span> <span class="n">n</span><span class="p">:</span> <span class="n">grad_to_var</span><span class="o">.</span><span class="n">has_key</span><span class="p">(</span><span class="n">n</span><span class="p">),</span>
                         <span class="n">op_desc</span><span class="o">.</span><span class="n">output_arg_names</span><span class="p">()):</span>
        <span class="n">fwd_var</span> <span class="o">=</span> <span class="n">block</span><span class="o">.</span><span class="n">var_recursive</span><span class="p">(</span><span class="n">grad_to_var</span><span class="p">[</span><span class="n">grad_n</span><span class="p">])</span>
        <span class="n">error_clip</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">fwd_var</span><span class="p">,</span> <span class="s2">&quot;error_clip&quot;</span><span class="p">,</span> <span class="bp">None</span><span class="p">)</span>
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        <span class="k">if</span> <span class="ow">not</span> <span class="p">(</span><span class="n">error_clip</span> <span class="ow">is</span> <span class="bp">None</span> <span class="ow">or</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">error_clip</span><span class="p">,</span>
                                                 <span class="n">BaseErrorClipAttr</span><span class="p">)):</span>
            <span class="k">raise</span> <span class="ne">TypeError</span><span class="p">(</span>
                <span class="s2">&quot;Variable&#39;s error_clip should be an instance of BaseErrorClipAttr or None.&quot;</span>
            <span class="p">)</span>
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        <span class="k">if</span> <span class="n">error_clip</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span><span class="p">:</span>
            <span class="n">error_clip</span><span class="o">.</span><span class="n">append_clip_op</span><span class="p">(</span><span class="n">block</span><span class="p">,</span> <span class="n">grad_n</span><span class="p">)</span>
</pre></div>
</div>
<p>This function takes a <code class="docutils literal"><span class="pre">block</span></code> and a <code class="docutils literal"><span class="pre">context</span></code>(which is actually a grad_to_var map) as inputs. It checks each output of the last <code class="docutils literal"><span class="pre">OpDesc</span></code> in the <code class="docutils literal"><span class="pre">block</span></code>. Notice that the last <code class="docutils literal"><span class="pre">OpDesc</span></code> of the <code class="docutils literal"><span class="pre">block</span></code> must be a <code class="docutils literal"><span class="pre">grad_op</span></code> and its outputs must be some forward variables&#8217; gradients. If an output gradient&#8217;s corresponding forward variable has an attribute of <code class="docutils literal"><span class="pre">error_clip</span></code>, <code class="docutils literal"><span class="pre">error_clip_callback</span></code> will call the <code class="docutils literal"><span class="pre">error_clip</span></code>&#8216;s <code class="docutils literal"><span class="pre">append_clip_op</span></code> function to append the required <code class="docutils literal"><span class="pre">clip_op</span></code> into the <code class="docutils literal"><span class="pre">block</span></code>.</p>
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