title: "Error using roialign" when training a Mask R-CNN model
body: <p>I am trying to train a <code>maskrcnn</code> model with <code>trainMaskRCNN()</code>, but get the following error after a number of iterations:</p>
  <pre><code>Warning: Empty Proposals. Skipping training step 
  &gt; In maskrcnn/regionProposal (line 83)
  In maskrcnn/forward (line 547)
  In images.dltrain.internal.SerialTrainer&gt;modelGradients (line 133)
  In deep.internal.dlfeval (line 17)
  In dlfeval (line 40)
  In images.dltrain.internal/SerialTrainer/fit (line 76)
  In images.dltrain.internal.dltrain (line 102)
  In trainMaskRCNN (line 240)
  In fibers_train (line 50) 
  Error using roialign
       outFeatures = roialign(X, squeeze(boxes), poolSize, 'ROIScale', 1);
                                 ↑
  Invalid argument at position 2. Value must not be empty.
  
  Error in maskrcnn/roiAlignPooling (line 19)
      outFeatures = roialign(X, squeeze(boxes), poolSize, 'ROIScale', 1);
  
  Error in maskrcnn/forward (line 550)
              dlPooled = roiAlignPooling(obj, dlX, dlProposals, obj.PoolSize);
  
  Error in images.dltrain.internal.SerialTrainer&gt;modelGradients (line 133)
  [networkOutputs{:},state] = forward(net,inputs{:});
  
  Error in deep.internal.dlfeval (line 17)
  [varargout{1:nargout}] = fun(x{:});
  
  Error in dlfeval (line 40)
      [varargout{1:nargout}] = deep.internal.dlfeval(fun,varargin{:});
  
  Error in images.dltrain.internal.SerialTrainer/fit (line 76)
                     [loss,grad,state,networkOutputs,lossData] = dlfeval(@modelGradients,self.Network,self.LossFcn,...
  
  Error in images.dltrain.internal.dltrain (line 102)
  net = fit(networkTrainer);
  
  Error in trainMaskRCNN (line 240)
      [network,info] = images.dltrain.internal.dltrain(mbqTrain,network,options,lossFcn,metrics,'Loss', 'ExperimentMonitor',params.ExperimentMonitor);
  
  Error in fibers_train (line 50)
  [net,info] = trainMaskRCNN(ds,net,options); %,FreezeSubNetwork=&quot;backbone&quot;
  </code></pre>
  <p>I don't really have any idea of what could be the cause of this. Any help?</p>