avoid aten dispatch shadowing type with variable (#121659)
Summary:
`DECLARE_DISPATCH` is shadowing the variable data with the data type:
`extern TORCH_API struct name name` -> `extern TORCH_API struct gemm_stub gemm_stub` for instance.
This is probably dangerous behavior to rely on, as the compiler needs to always resolve to type and/or data based on context. Previous macro fails with VS2022.
Test Plan: `buck2 build arvr/mode/win/vs2022/cpp20/opt //xplat/caffe2:aten_pow_ovrsource`
Differential Revision: D54699849
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121659
Approved by: https://github.com/albanD
