[GitHub] ptrendx opened a new pull request #14209: Fix SliceLikeBackward

[GitBox <gi...@apache.org>]

ptrendx opened a new pull request #14209: Fix SliceLikeBackward
URL: https://github.com/apache/incubator-mxnet/pull/14209
 
 
   ## Description ##
   
   This PR fixes a bug in `slice_like` operator, where, if the first input is constant, the backward pass of this operator would not fill the gradient to the second input with zeros, instead passing random memory contents.
   
   ## Checklist ##
   ### Essentials ###
   Please feel free to remove inapplicable items for your PR.
   - [x] Changes are complete (i.e. I finished coding on this PR)
   - [x] To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
   
   ### Changes ###
   - [x] Moved the handling of the second input before the early exit for the first input
   
   ## Comments ##
   - This issue was encountered while working with GluonCV's SSD model, where `slice_like` is used in anchor generator (https://github.com/dmlc/gluon-cv/blob/master/gluoncv/model_zoo/ssd/anchor.py#L70):
   ```
   a = F.slice_like(anchors, x * 0, axes=(2, 3))
   ```
   where the work around was to multiply the second input by 0 (effectively zeroing the gradient). With this PR, this work around is not needed anymore (and it is also not fully safe - NaNs and Infs in the memory returned from SliceLikeBackward would produce NaNs when multiplied by 0 and so would still break the training). @zhreshold FYI
   
   @eric-haibin-lin 

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