Add initial support for differentiable optimizers (#80938)
Adds the `differentiable` argument, a method for updating parameters in an existing optimizer, and a template for testing the differentiability of multiple optimizers.
This is all based in discussions with @albanD & @jbschlosser
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80938
Approved by: https://github.com/albanD
