Track gradients of activations in HistogramLogger

[callumm-graphcore]

Is your feature request related to a problem? Please describe.
I don't think we track these at the moment

Describe the solution you'd like
We can do it using register_backward_hook, presumably

Describe alternatives you've considered
We could just not do it and hope for the best idk

Additional context
No

[callumm-graphcore]

There might be complications arising from the fact that we take gradients of gradients during training (calculate gradients to compute the forces, then compare predicted forces to actual forces, then calculate gradients with respect to the forces loss)