Core: Multiple input/param gradient modification

[chr5tphr]

• change the core Hook to support the modification of multiple inputs and params
• for this, now each input and parameter that requires a gradient will be hooked, and a backward, which is aware of which the current 'sink' is, will be called for each
• use View instead of custom Identity to produce a .grad_fn

Note:

• this may be a breaking change for custom hooks based on the old implementation

TODO:

• finish implementation:
  • parameters have no grad_fn, and we cannot simply overwrite them with a view; hooking directly with tensor hooks is problematic when the parameters are used in different functions
  • there may be potentially a better approach than calling the backward function once per 'sink', although the current implementation may allow for better modularity - multiple outputs are still not supported, it may be worth to think how to do it, however, it may also be better to do this at a later stage
• implement tests
  • new tests for the new functionality: multiple inputs and params in hooks
  • fix old tests that assume the use of Identity and are not sink-aware
• add documentation