v0.8.21

NNlib v0.8.21

Diff since v0.8.20

Merged pull requests:

• Add (log)softmax benchmarks (#491) (@ToucheSir)
• make NNlibCUDA an extension (#492) (@CarloLucibello)
• Fix nthreads on 1.9 (failure in the presence of interactive threads) (#496) (@IanButterworth)
• backports v0.8.21 (#498) (@CarloLucibello)