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

functorch/__init__.py

@@ -4,7 +4,6 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 import torch
-from . import _C
 
 # Top-level APIs. Please think carefully before adding something to the
 # top-level namespace: