stephenrawls commented on issue #14208: Add support for fast variable-length 
LSTM
URL: https://github.com/apache/incubator-mxnet/pull/14208#issuecomment-465408946
 
 
   @szha maybe this deserves another PR but I also noticed that cuDNN supports 
both sequence-major and batch-major sequences:
   
   From: 
https://docs.nvidia.com/deeplearning/sdk/cudnn-developer-guide/index.html#cudnnRNNForwardInferenceEx
   ```
   With unpacked layout, both sequence major (i.e. time major) and batch major 
are supported
   ```
   
   Currently gluon is supporting batch-major sequences by calling swapaxes:
   
https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/gluon/rnn/rnn_layer.py#L241-L242
   
   This ends up calling mshadow's swapaxis function, which I *assume* must be 
doing a copy (?) but to be honest I couldn't totally follow what the code is 
doing at a quick glance:
   
https://github.com/dmlc/mshadow/blob/3dc80815d965b56b9a975dc27229361955bf66fe/mshadow/extension/swapaxis.h
 
   
   If it *is* doing a copy for swapaxes, it would be preferable for Gluon to 
just let cudnn handle the batch-major layout directly w/o calling swapaxes.

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