[doc] Update einsum documentation.

[fadel]

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.

[facebook-github-bot]

@soumith is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[soumith]

thanks!

[Dominik1123]

The docstring still states that operands should be of type list of Tensors. According to PEP 484 on type hints, packed arguments should be annotated such that the hint applies to each single argument, i.e. *operands: Tensor. Hence the same should apply to doc string type hints, i.e. operands should be declared Tensor instead of list of Tensors.

With the current docstring version, IDEs such as PyCharm issue warnings that the remaining positional arguments to einsum should be of type List instead of Tensor because they infer this type from the docstring.