Update einsum documentation. (#16323)

Update einsum documentation. (#16323)

Summary:
The documentation stated that operands to einsum should be a list of Tensors, not individual arguments. The function, however, now accepts individual arguments for each Tensor operand *and* a single argument consisting of a list of Tensors. The documentation was updated to reflect this change.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16323

Differential Revision: D13832647

Pulled By: soumith

fbshipit-source-id: c01c2b350f47674d3170337f493b0ee2ea381b3f