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/cast.rs

@@ -0,0 +1,28 @@
+// Skip on metal - F64 not supported
+#![cfg(all(feature = "std", not(feature = "metal")))]
+
+use super::*;
+use burn_backend_tests::might_panic;
+use burn_tensor::{DType, Tensor, TensorData};
+
+#[might_panic(reason = "Unsupported precision for fusion")]
+#[test]
+fn cast_keeps_gradient_flow() {
+    let device = Default::default();
+
+    let x = Tensor::<TestAutodiffBackend, 2>::from_data(
+        TensorData::from([[1.0, 2.0], [3.0, 4.0]]),
+        &device,
+    )
+    .require_grad();
+
+    let y = x.clone().cast(DType::F64);
+    let z = y.sum();
+
+    let grads = z.backward();
+    let grad_x = x.grad(&grads).unwrap();
+
+    grad_x
+        .to_data()
+        .assert_eq(&TensorData::from([[1., 1.], [1., 1.]]), false);
+}