Extend custom derivatives functionality to facilitate custom _forw functions.

[parth-07]

In reverse-mode AD, Clad generates forward pass functions (_forw) to facilitate correct differentiation of functions returning a pointer or a reference. For more information about forward pass functions, please refer #425.

Clad may fail to generate the forward pass function in some cases and thus it would be beneficial to support custom _forw functions in the same way we support custom pushforwards and custom pullbacks.

[gojakuch]

solved in #1037