Better way to set retain_graph

[AntixK]

Is there a better way to set retain_graph, especially when using two optimizers?

I have read the issue #356 and the corresponding fix, to set it by overriding the backward function.
However, this becomes messy especially when more than 1 optimizers are used, as the function doesn't have the optimizer_idx as an argument.

There are workarounds like naming the optimizer object and using that to set/reset the retain graph, but passing optimizer_idx would be great.

This shouldn't be difficult as the opt_idx variable is available inside the loop in which the backward function is called

• Version [0.6.0]

[williamFalcon]

good suggestion. a pr for this would be great!