Add examples using operators.

src/continuity/pde/grad.py

@@ -53,6 +53,13 @@ def forward(self, x: Tensor, u: Tensor, y: Optional[Tensor] = None) -> Tensor:
 def grad(u: Callable[[Tensor], Tensor]) -> Callable[[Tensor], Tensor]:
     """Compute the gradient of a function.
 
+    Example:
+        Computing the gradient of the output function of an operator:
+        ```python
+        v = lambda y: operator(x, u, y)
+        g = grad(v)(y)
+        ```
+
     Args:
         u: Function to compute the gradient of.
 
@@ -92,6 +99,13 @@ def forward(self, x: Tensor, u: Tensor, y: Optional[Tensor] = None) -> Tensor:
 def div(u: Callable[[Tensor], Tensor]) -> Callable[[Tensor], Tensor]:
     """Compute the divergence of a function.
 
+    Example:
+        Computing the divergence of the output function of an operator:
+        ```python
+        v = lambda y: operator(x, u, y)
+        d = div(v)(y)
+        ```
+
     Args:
         u: Function to compute the divergence of.