I see your point. But according to the docs, these two methods should not make any difference, should they?
ignore_index (int, optional) – 
Specifies a target value that is ignored 
and does not contribute to the input gradient. 
When size_average is True, 
the loss is averaged over non-ignored targets.

Setting ignore_index to the padding value of the targets, their losses will be ignored. It also says the with size_average=True, the mean is based on the non - ignored losses.
However, I’m curious how this behaves with reduction as size_average is going to be deprecated.
Regards,
Unity05