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

src/bin/sample/main.rs

@@ -14,7 +14,7 @@ cfg_if::cfg_if! {
     if #[cfg(feature = "wgpu-backend")] {
         use burn_wgpu::{Wgpu, WgpuDevice};
     } else {
-        use burn_tch::{LibTorch, LibTorchDevice};
+        use burn_ndarray::NdArrayDevice;
     }
 }
 
@@ -61,24 +61,8 @@ fn main() {
             type Backend = Wgpu;
             let device = WgpuDevice::BestAvailable;
         } else {
-            type Backend = LibTorch<f32>;
-
-            let device = if let Some(dev_str) = device_arg {
-                match dev_str.to_lowercase().as_str() {
-                    "cpu" => LibTorchDevice::Cpu,
-                    "mps" => LibTorchDevice::Mps,
-                    s if s.starts_with("cuda") => {
-                        let idx = s[4..].parse().unwrap_or(0);
-                        LibTorchDevice::Cuda(idx)
-                    }
-                    _ => {
-                        eprintln!("Unknown device: {}", dev_str);
-                        process::exit(1);
-                    }
-                }
-            } else {
-                LibTorchDevice::Cuda(0)
-            };
+            type Backend = burn::backend::ndarray::NdArray<f32>;
+            let device = NdArrayDevice::Cpu;
         }
     }