Broken link in Tuning Guide

[bwalshe]

The file /recipes_source/recipes/tuning_guide.py contains a link to the documentation for torch.optim.Optimizer.zero_grad(), but this link does not work for me.

Proposed change:

• Current link: https://pytorch.org/docs/master/optim.html#torch.optim.Optimizer.zero_grad
• New link: https://pytorch.org/docs/master/generated/torch.optim.Optimizer.zero_grad.html