Update 'einsum' docstring to conform to PEP-484 (#27563)
Summary:
[PEP-484](https://www.python.org/dev/peps/pep-0484/#arbitrary-argument-lists-and-default-argument-values) specifies that arbitrary argument lists, here `*operands`, should be annotated with the type of the single arguments, i.e. not indicating that the whole thing is wrapped into a `list` (which is a Python internal anyway). The previous docstring caused problems with type checkers for IDEs such as PyCharm ([see here](https://youtrack.jetbrains.com/issue/PY-38035)).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27563
Differential Revision: D17904748
Pulled By: soumith
fbshipit-source-id: 0a7fcbbb12e388e6fc40d48bf533652a96024757
