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Fix "calling median on empty tensor" issue in MR #1322
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Summary:
Language model metrics reporter occassionally throws "median on empty tensor", e.g: f182865817
why?
We used to check the cross entropy tensor != 0 before calling mean/median on it, to exclude the calculation of the padding positions which is just 0, which possibly results in an empty tensor.
This diff checks the tensor posisions against pad_index instead of using 0.0:a batch can have close to 0 CE, or the predict word index happened to be pad_index.
Also a failed safe is added if the tensor[target != self.pad_index] is indeed empty.
Differential Revision: D21081502