Better default for number of threads
Better default for number of threads

Summary: Pull Request resolved: https://github.com/facebookresearch/fastText/pull/659

Reviewed By: piotr-bojanowski

Differential Revision: D13318793

Pulled By: Celebio

fbshipit-source-id: 3b8fd28172b0291b2df93a526de630b01df680e8

facebookresearch/fastText,facebookresearch/fastText,facebookresearch/fastText,facebookresearch/fastText,facebookresearch/fastText,facebookresearch/fastText