[pytorch] add fallthrough variable kernel for C10_MOBILE (#35491)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35491
The goal of this diff is to avoid having to set AutoNonVariableTypeMode guard
in client code that uses custom mobile build. The guard was necessary because
custom mobile build might not include variable kernels, in which AutoNonVariableTypeMode
guard is usually set. It's hard to enforce all callsites to follow this rule, so
we make this change to simplify it.
Another goal of the diff is to not break FL where real variable kernels are
registered.
ghstack-source-id: 100944553
Test Plan:
- With stacked diff, tested lite-trainer with MnistModel:
```
buck run xplat/caffe2/fb/lite_trainer:lite_trainer \
-c pt.disable_gen_tracing=1 \
-- --model=/home/liujiakai/ptmodels/MnistModel.bc
```
- Will test with the papaya sample app.
Differential Revision: D20643627
fbshipit-source-id: 37ea937919259c183809c2b7acab0741eff84d33
