add len to nativeResolver (#15488)
Summary:
(otherwise len is not resolvable using torch::jit::compile)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15488
Differential Revision: D13539991
Pulled By: zdevito
fbshipit-source-id: 3ba85fa7b1adb163f9229c568f7997d22321903d
