Only use ::dfdx as a path entrypoint on code generation

- Re-export the `self` crate as `dfdx`.
  - See [related question](https://users.rust-lang.org/t/how-to-express-crate-path-in-procedural-macros/91274/10).
  - Does the same for `dfdx-core`.
- Change the generated code so they always have the `::dfdx` path entrypoint.
  - User apps no longer need to depend on `dfdx-core` or `dfdx-derives` directly.
- Re-export `safetensors`.
  - User apps no longer need to depend on `safetensors` directly (as in, this is no longer required by the `SaveSafeTensors` and `LoadSafeTensors` derivations).

dfdx-core/src/lib.rs

@@ -110,6 +110,7 @@
 extern crate alloc;
 #[cfg(all(feature = "no-std", not(feature = "std")))]
 extern crate no_std_compat as std;
+extern crate self as dfdx_core;
 
 pub mod data;
 pub mod dtypes;