Updating the custom_differentiation tutorial for the S4TF 0.5 release. (#270) (#284)
Updating the custom_differentiation tutorial for the S4TF 0.5 release. (#270) (#284)

- Rename `withGradient` to `withDerivative`.
- Remove deprecated `(a|A)llDifferentiableVariables`.
- Update outputs of custom_differentiation.ipynb.
  - New version has fixed execution_count in some cells.
  - New version has different computation results.
tensorflow/swift,tensorflow/swift