Numpy einsum missing type for common case

[dabacon]

Description of the issue

Numpy does not currently have an override for the case when the type is of the form operator, indices, operator, indices, etc. https://github.com/numpy/numpy/issues/21978

When this is added to numpy and we are upgraded to a version that captures this, we should fix the places where we have had ignore mypy errors.

Cirq version v0.15

[dabacon]

Wow numpy already has submitted a fix, will need to wait for our numpy version to catch up.