`torch.einsum` docs don't mention that `opt_einsum` must be installed separately

[bmillwood]

📚 The doc issue

Neither the torch.einsum doc page nor the torch.backends.opt_einsum page mentions the necessary / sufficient conditions to make the backend available.

Before I looked into it, I imagined that any of these things could be true (now I think they are all false):

• opt_einsum is a hard dependency of pytorch,
• the code in opt_einsum is just included into pytorch, so it's always available and it doesn't matter what package you have installed, but it might be out of date if opt_einsum does something new,
• the optimizations will only apply if you import opt_einsum in your program
  • maybe even you have to import opt_einsum before you load torch, or before any of your other imports load it?

Suggest a potential alternative/fix

Mention that:

• it's necessary to install the opt_einsum package yourself separately (perhaps link to https://optimized-einsum.readthedocs.io/en/stable/install.html )
• it's doesn't matter if you import opt_einsum in your code, pytorch will import it itself if it exists

Minor bonus: the note in the einsum docs mentions torch.backends.opt_einsum, would be convenient if that note were a link

cc @svekars @brycebortree

[soulitzer]

cc @janeyx99