Add THP_API to THPGenerator_Wrap (#35194)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35194
Fixes:
```
error LNK2001: unresolved external symbol "struct _object * __cdecl THPGenerator_Wrap(struct at::Generator)" (?THPGenerator_Wrap@YAPEAU_object@UGenerator@at@@Z)
build\lib.win-amd64-3.6\torch_test_cpp_extension\rng.cp36-win_amd64.pyd : fatal error LNK1120: 1 unresolved externals
```
I forgot it, my fault
Test Plan: Imported from OSS
Differential Revision: D20591604
Pulled By: pbelevich
fbshipit-source-id: e8986948fb50aec50db99a72ad112702cbbe831f
