Fix broken docs (#124940)

These were causing doctest to be unhappy.

In particular the doc from #124496 caused #124771 to fail "trunk / win-vs2019-cpu-py3 / test" to fail when pushing. Not sure why it wasn't a problem on the original PR.

Testing:

`./test/run_doctests.sh`:
  before:
```
=== 4 warnings in 11.21 seconds ===
```
  after:
```
===  in 11.11 seconds ===
```

Pull Request resolved: https://github.com/pytorch/pytorch/pull/124940
Approved by: https://github.com/zou3519, https://github.com/atalman, https://github.com/huydhn

torch/library.py

@@ -849,11 +849,11 @@ def opcheck(
         >>> def _(x, y):
         >>>     return torch.empty_like(x)
         >>>
-        >>> def setup_context(ctx, inputs, output)
+        >>> def setup_context(ctx, inputs, output):
         >>>     y, = inputs
         >>>     ctx.y = y
         >>>
-        >>> def backward(ctx, grad)
+        >>> def backward(ctx, grad):
         >>>     return grad * ctx.y, None
         >>>
         >>> numpy_sin.register_autograd(backward, setup_context=setup_context)