Fix caffe2 import tools.codegen (#50353)
Summary:
Using `insert` instead of `append` to add torch root directory to `sys.path`, to fix `ModuleNotFoundError: No module named 'tools.codegen'`, as mentioned in https://github.com/pytorch/pytorch/issues/47553
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50353
Reviewed By: mrshenli
Differential Revision: D25893827
Pulled By: ezyang
fbshipit-source-id: 841e28898fee5502495f3890801b49d9b442f9d6
