Propagate zero derivative indicator

[dfdx]

Motivating example is RNN which returns a tuple of (last_hidden, all_hiddens), but usually only one of these is actually used in further computations. In such settings, gradient w.r.t. to non-target element of the tuple can be replaced with some kind of symbolic zero instead of actually allocated object.

[oxinabox]

This is one of the reasions ChainRulesCore has AbstractZero and its subtypes.

[dfdx]

Yep! Again, it's already integrated in #70, this issue will be closed as soon as that PR is merged.