Add opt-einsum to CI (#85574)
Depends on https://github.com/pytorch/pytorch/pull/84890.
This PR adds opt_einsum to CI, enabling path optimization for the multi-input case. It also updates the installation sites to install torch with einsum, but those are mostly to make sure it would work on the user's end (as opt-einsum would have already been installed in the docker or in prior set up steps).
This PR also updates the windows build_pytorch.bat script to use the same bdist_wheel and install commands as on Linux, replacing the `setup.py install` that'll become deprecated.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85574
Approved by: https://github.com/huydhn, https://github.com/soulitzer
