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/sigmoid.rs

@@ -0,0 +1,35 @@
+use super::*;
+use burn_tensor::Tolerance;
+use burn_tensor::{TensorData, activation};
+
+#[test]
+fn should_diff_sigmoid() {
+    let data = TensorData::from([0.8762]);
+
+    let device = Default::default();
+    let tensor_1 = TestAutodiffTensor::<1>::from_data(data, &device).require_grad();
+    let tensor_2 = activation::sigmoid(tensor_1.clone());
+    let grads = tensor_2.backward();
+
+    let grad = tensor_1.grad(&grads).unwrap();
+
+    let expected = TensorData::from([0.207549]);
+    grad.to_data()
+        .assert_approx_eq::<FloatElem>(&expected, Tolerance::default());
+}
+
+#[test]
+fn small_neg_val_should_not_cause_grad_overflow() {
+    let data = TensorData::from([-90.0]);
+
+    let device = Default::default();
+    let tensor_1 = TestAutodiffTensor::<1>::from_data(data, &device).require_grad();
+    let tensor_2 = activation::sigmoid(tensor_1.clone());
+    let grads = tensor_2.backward();
+
+    let grad = tensor_1.grad(&grads).unwrap();
+
+    let expected = TensorData::from([0.0]);
+    grad.to_data()
+        .assert_approx_eq::<FloatElem>(&expected, Tolerance::default());
+}