Default Implementation

Methods

The default implementation favors composition by implementing a method in term of another, especifically if follows this call graph:

     summary            predict         evalutate                            fit              init
        ⬇️                  ⬇️                ⬇️                                 ⬇️                 ⬇️
call_summary_step    call_pred_step   call_test_step                   call_train_step   call_init_step
        ⬇️                  ⬇️                ⬇️                                 ⬇️                 ⬇️
 summary_step    ➡️     pred_step    ⬅   test_step    ⬅   grad_step   ⬅   train_step    ⬅    init_step

This structure allows you to for example override test_step and still be able to use use fit since train_step (called by fit) will call your test_step via grad_step. It also means that if you implement test_step but not pred_step there is a high chance both predict and summary will not work.

call_* methods

The call_<method> method family are entrypoints that usually just redirect to their inputs to <method>, you choose to override these if you need to perform some some computation only when method in question is the entry point i.e. when its not called by other methods in the bottom path. For example if you want to change the behavior of evaluate without affecting the behavior of fit while preserving most of the default implementation you can override call_step_step to do the corresponding adjustments and then call test_step. Since train_step does not depend on call_step_step then the change will manifest during evaluate but not during fit.