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/gradients.rs

@@ -0,0 +1,24 @@
+use super::*;
+use burn_tensor::{Distribution, activation};
+
+#[test]
+fn should_update_tensor_when_grad_replace() {
+    let device = Default::default();
+    let tensor_1 =
+        TestAutodiffTensor::<2>::random([32, 32], Distribution::Default, &device).require_grad();
+    let tensor_2 = TestAutodiffTensor::random([32, 32], Distribution::Default, &device);
+
+    let x = tensor_1.clone().matmul(activation::gelu(tensor_2));
+    let mut grads = x.backward();
+
+    let grad_1 = tensor_1.grad(&grads).unwrap();
+
+    let grad_1_updated =
+        TestAutodiffTensor::random([32, 32], Distribution::Default, &device).require_grad();
+    tensor_1.grad_replace(&mut grads, grad_1_updated.clone().inner());
+
+    let grad_1_new = tensor_1.grad(&grads).unwrap();
+
+    assert_ne!(grad_1_new.to_data(), grad_1.into_data());
+    assert_eq!(grad_1_new.into_data(), grad_1_updated.into_data());
+}