Annotate SamplingConfig.seed as optional

[kmonte]

This may actually get set as None if you see here 1, 2

and this fields usages are gated behind a is None check 3 4

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}