Stop loading functorch._C unless torchdim is needed (#100491)
Just a small optimization. This PR changes it so that import of
functorch.dim ends up loading functorch._C (which is entirely composed
of torchdim APIs)
Test Plan:
- existing tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100491
Approved by: https://github.com/Chillee, https://github.com/kshitij12345
