feat: migrate to pure Rust backend with WGPU/Vulkan support

- Remove libtorch dependency and related features
- Switch to ndarray backend as default for CPU execution
- Update README to reflect new WGPU/Vulkan backend usage
- Simplify device selection to use only CPU backend
- Enable WGPU backend via feature flag for GPU acceleration

Cargo.toml

@@ -7,14 +7,12 @@ edition = "2021"
 
 [features]
 wgpu-backend = ["burn-wgpu"]
+default = ["wgpu-backend"]
 
 [dependencies]
 burn = "0.20.1"
-burn-ndarray = "0.20.1"
-burn-tch = "0.20.1"
 burn-autodiff = "0.20.1"
 burn-wgpu = { version = "0.20.1", optional = true }
-tch = "0.22.0"
 serde = {version = "1.0.171", features = ["std", "derive"]}
 npy = "0.4.0"
 num-traits = "0.2.15"