Ignore index functionality missing in MixUp

[vedantdalimkar]

I think the current implementation of MixUp does not handle ignore_index indices in the labels. Is it supposed to work this way?

If any change is required, I would be happy to work on a PR.

[AntoineSimoulin]

@vedantdalimkar thanks for posting! Can you give more detail about the bug regarding the ignore_index indices in the labels and propose a short code snippet on the current state and how it differs from the expected behavior?