Merge hooks that save layer inputs and compute H_K, H_C

[f-dangel]

After merging #63, the accumulation of H_K, H_C will be done with a tensor hook that gets installed onto the output of a layer. We also have a second hook which saves the input to a layer into a .inputs dictionary which acts like a global variable.

Proposal: Instead of saving the input to a layer into SINGD.layers, directly pass it to the tensor hook that computes H_K, H_C. This

1. Eliminates a 'global' variable (SINGD.inputs)
2. Reduces the number of hooks (and related boilerplate code like checking if .step matches the update frequency) from 2 to 1