Move input array checks out of autograd example

[jwallwork23]

Towards #211.
Follows #225.

This PR cuts down the amount of testing done in the autograd example. In particular, it moves the checks that input arrays aren't modified into the unit tests.

The PR also adds a missing summary for the autograd example in examples/README.md.

[jwallwork23]

Just pushed a couple of commits because I realised that the cleanup subroutines weren't being used everywhere. Further, the shape checks were redundant because shape checking is already done in the assert_allclose subroutines.