[CMake] When should we use static lib and when shared

### 🐛 Describe the bug

We can definitely split backends and ops into its own .so library.

Right now we don't have a clear guideline when to use which. 

We also have something like 

```
 if(CMAKE_TOOLCHAIN_IOS
    OR CMAKE_TOOLCHAIN_ANDROID
    OR APPLE
 )
```
repeating.


### Versions

main
a325c56864c5b25630c1ebda03c8f16a96a02a19

cc @larryliu0820 @jathu