PyTorch: [Stable-ABI] Add torch::stable::masked_select. (#198048)

Executive Summary

Exposes masked_select through the stable ABI as torch::stable::masked_select . torchvision's stable-ABI currently utilizes a local masked_select helper . Summary Adds torch::stable::masked_select , which calls aten::masked_select through the dispatcher and returns a 1-D tensor of the elements of self where mask is true. The wrapper has the same shape as torch::stable::index_select .

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