Hi Diogo -
We currently have a subset of support for numpy indexing but what I believe you are referring to is Boolean array indexing as described here: https://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#boolean-array-indexing 701.
The closest PyTorch equivalent is masked select, as described here: http://pytorch.org/docs/master/torch.html?highlight=masked_select#torch.masked_select 2.5k. If you could instead generate your mask in the ByteTensor format described there, you could achieve what you want.