[Lite Trainer] Add necessary registrations for MNIST model (#33717)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33717
Because of the special treatment of operator names for lite interpreter, all the operators used in lite interpreter are still prepended by "_". Add the necessary registrations for MNIST model. All the ops with autograd capability are included in torch_mobile_train. After rebase the selective build from D19649074 can be utilized to strip the unused ops.
Note that this diff is for feasibility test. The training accuracy are not covered in the test.
ghstack-source-id: 97780066
Test Plan:
```
buck run xplat/caffe2/fb/lite_trainer:lite_trainer -c pt.disable_gen_tracing=1 -c pt.static_dispatch=0 -- --model=/path/MnistModel.bc
```
{F227898221}
Reviewed By: dreiss
Differential Revision: D19743201
fbshipit-source-id: cacadd76f3729faa0018d147a69466bbf54312fd
