fix path in examples

Author: refraction-ray

examples/nested_vmap_grad.py

@@ -2,6 +2,10 @@
 demonstration of vmap + grad like API
 """
 
+import os
+
+sys.path.insert(0, "../")
+
 import tensorcircuit as tc
 
 # See issues in https://github.com/tencent-quantum-lab/tensorcircuit/issues/229#issuecomment-2600773780