backward() for ActivationFunction

[grantbruer]

1. Added backward() for ActivationFunction. It calls the user-defined backward function using the output for invertible functions and the input for non-invertible functions.
2. Fixed ExpClampLayer activation. The forward computation didn't match the inverse and gradient.
3. Updated some failing gradient tests to use sigmoid so that they don't rely on the gradient of relu at x = 0.

[grantbruer]

The previous limitations were as follows:

• ResidualBlock requires non-invertible activations.
• CouplingLayerBasic requires invertible activation.
• CouplingLayerGlow requires invertible activation.
• ConditionalLayerGlow requires invertible activation.

With this PR, each layer should be able to use arbitrary activation functions, with the stipulation that the coupling layers need positive activations.

In this PR, I didn't add tests to make sure these work correctly, although I have run more tests in a different PR to make sure these changes are correct. If someone else wants to add commits with unit tests for these behaviors, please do. I don't think I'll have time for a couple months.