Add quick little first test to check for grad_fn in results

test/test_differentiable.py

@@ -0,0 +1,26 @@
+import torch
+
+import cheetah
+
+
+def test_simple_quadrupole():
+    """
+    Simple test on a [D, Q, D] lattice with the qudrupole's k1 requiring grad, checking
+    if PyTorch tracked a grad_fn into the outgoing beam.
+    """
+    segment = cheetah.Segment(
+        [
+            cheetah.Drift(torch.tensor(1.0)),
+            cheetah.Quadrupole(torch.tensor(0.2), k1=torch.tensor(1.0), name="my_quad"),
+            cheetah.Drift(1.0),
+        ]
+    )
+    incoming_beam = cheetah.ParticleBeam.from_astra(
+        "benchmark/astra/ACHIP_EA1_2021.1351.001"
+    )
+
+    segment.my_quad.k1.requires_grad = True
+
+    outgoing_beam = segment.track(incoming_beam)
+
+    assert hasattr(outgoing_beam, "grad_fn")