feat: update workspace paths and enhance gitignore

- Updated stablediffusion crate path from "../stable-diffusion-burn" to "./crates/stable-diffusion-burn" for proper workspace resolution
- Enhanced .gitignore to include generated model files (.mpk, .pt, .bin, .safetensors, .ckpt) and user_data directory
- Added Cargo.lock to gitignore with appropriate comment
- Reorganized IDE files section in gitignore for better clarity
- Added newline at end of file for proper formatting

crates/stable-diffusion-burn/burn-crates/burn-backend-tests/tests/autodiff/backward.rs

@@ -0,0 +1,24 @@
+use super::*;
+use burn_tensor::{Int, Tensor, TensorData, module::embedding};
+
+#[test]
+fn test_embedding_backward() {
+    let weights = TensorData::from([[0.0, 1.0, 2.0], [3.0, 4.0, 5.0]]);
+    let indices = TensorData::from([[0, 1], [1, 1]]);
+    let x = TensorData::from([
+        [[1.0, 2.0], [4.0, 5.0], [3.0, 4.0]],
+        [[4.0, 5.0], [8.0, 5.0], [1.0, 9.0]],
+    ]);
+    let device = Default::default();
+    let weights = Tensor::<TestAutodiffBackend, 2>::from_data(weights, &device).require_grad();
+    let indices = Tensor::<TestAutodiffBackend, 2, Int>::from_data(indices, &device);
+    let x = Tensor::<TestAutodiffBackend, 3>::from_data(x, &device).require_grad();
+
+    let output = embedding(weights.clone(), indices);
+    let output = output.matmul(x);
+    let grads = output.backward();
+
+    let grad = weights.grad(&grads).unwrap();
+    grad.to_data()
+        .assert_eq(&TensorData::from([[3., 9., 7.], [21., 35., 27.]]), false);
+}