[GitHub] wkcn commented on issue #11254: add blocklist

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wkcn commented on issue #11254: add blocklist
URL: https://github.com/apache/incubator-mxnet/pull/11254#issuecomment-416561560
 
 
   @chinakook Thanks.
   I don't know it.
   `mxnet.gluon.nn.Sequential` is a serial structure.
   
[python/mxnet/gluon/nn/basic_layers.py#L51](https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/gluon/nn/basic_layers.py#L51)
   ```python
   class Sequential(Block):
       [...]
       def forward(self, x):
           for block in self._children.values():
               x = block(x)
           return x
   ```
   How to use `Sequential` to implement a parallel structure like multi-column 
CNN?
   [Single-Image Crowd Counting via Multi-Column Convolutional Neural Network 
[Page:4]](https://www.cv-foundation.org/openaccess/content_cvpr_2016/papers/Zhang_Single-Image_Crowd_Counting_CVPR_2016_paper.pdf)
   
   I think `BlockList` is useful.
   e.g.
   ```python
   class MCCNN(nn.HybridBlock):
       def __init__(self, num_filter, columns, aggregator, **kwargs):
           super(MCCNN, self).__init__(**kwargs)
           with self.name_scope():
               self.column_blocks = BlockList(self, [get_column(num_filter, 
kernel_size, dilations) for kernel_size, dilations in columns])
               self.aggregator = get_aggregator(num_filter, aggregator[0], 
aggregator[1]) 
               self.conv1x1 = nn.Conv2D(channels = 1, kernel_size = (1, 1), 
activation = 'relu')
       def hybrid_forward(self, F, x):
           column_outputs = [c(x) for c in self.column_blocks]
           x = F.concat(*column_outputs, dim = 1)
           x = self.aggregator(x)
           x = self.conv1x1(x)
           x = F.squeeze(x, axis = 1)
           return x
   ```
   
   And here is my BlockList implementation.
   ```python
   def BlockList(self, block_list):
       block_names = []
       for block in block_list:
           assert not hasattr(self, block.name)
           setattr(self, block.name, block)
           block_names.append(block.name)
       class _BlockList:
           def __init__(self, block_names, parent):
               self.block_names = block_names
               self.parent = parent
           def __getitem__(self, i):
               return getattr(self.parent, self.block_names[i])
       return _BlockList(block_names, self)
   ```

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