[Pytorch][3/4 Static dispatch] Move static dispatch logic to Operators.cpp (#76058)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76058
Move static dispatch logic into Operators.cpp instead of having them duplicated in Functions.h and TensorBody.h
ghstack-source-id: 154616593
Test Plan:
Builds lite predictor with lightweight_dispatch enabled
```
buck build --config pt.enable_lightweight_dispatch=1 --config pt.static_dispatch_backend="CPU;QuantizedCPU;CompositeExplicitAutograd" //xplat/caffe2/fb/lite_predictor:lite_predictor_flatbuffer
```
Reviewed By: larryliu0820, bdhirsh
Differential Revision: D35702065
fbshipit-source-id: 263fe77936b63e41c4efea953be518e81a2c55c6
(cherry picked from commit 2e71198bf952bd1e5562cac2d589791e05e0fb2f)
