Use ninja build as default for HIPExtensions (#38939)
Summary:
This PR adds the following changes:
1. It sets the default extension build to use ninja
2. Adds HIPCC flags to the host code compile string for ninja builds. This is needed when host code makes HIP API calls
cc: ezyang jeffdaily
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38939
Differential Revision: D21721905
Pulled By: ezyang
fbshipit-source-id: 75206838315a79850ecf86a78391a31ba5ee97cb
