Can we get rid of uses_backend

### 🚀 The feature, motivation and pitch

Usage: https://github.com/search?q=repo%3Apytorch%2Fexecutorch%20uses_backend&type=code

This can be done by client, using get_backend_name().

This could reduce binary size a bit? 🤔 

### Alternatives

_No response_

### Additional context

_No response_

### RFC (Optional)

_No response_