Optimize the performance of einsum's submodule sum.
Optimize the performance of einsum's submodule sum.

rgommers/numpy,pbrod/numpy,rgommers/numpy,pbrod/numpy,simongibbons/numpy,mattip/numpy,pdebuyl/numpy,anntzer/numpy,numpy/numpy,numpy/numpy,charris/numpy,pdebuyl/numpy,seberg/numpy,pdebuyl/numpy,mhvk/numpy,jakirkham/numpy,madphysicist/numpy,seberg/numpy,charris/numpy,anntzer/numpy,pbrod/numpy,charris/numpy,mattip/numpy,madphysicist/numpy,endolith/numpy,seberg/numpy,endolith/numpy,madphysicist/numpy,jakirkham/numpy,simongibbons/numpy,jakirkham/numpy,endolith/numpy,madphysicist/numpy,endolith/numpy,rgommers/numpy,anntzer/numpy,simongibbons/numpy,jakirkham/numpy,mattip/numpy,madphysicist/numpy,mhvk/numpy,jakirkham/numpy,pbrod/numpy,mhvk/numpy,charris/numpy,simongibbons/numpy,numpy/numpy,mattip/numpy,rgommers/numpy,simongibbons/numpy,mhvk/numpy,anntzer/numpy,pdebuyl/numpy,mhvk/numpy,seberg/numpy,numpy/numpy,pbrod/numpy