Hi there. Really enjoying trying out Postgres ML so far.

Using a test environment in the Cloud version, I tried to use 'first' as an option under test_sampling, per the docs:

However, it throws an error saying ERROR: invalid input value for enum pgml.sampling: "first"

Upon inspection of the source code, 'first' doesn't actually seem to be implemented (thanks to montanalow on Discord for flagging): https://github.com/postgresml/postgresml/blob/master/pgml-extension/src/orm/sampling.rs

Two potential resolutions:
1 (preferred): Implement 'first'
2) Update the docs to remove 'first' as an option

Thanks!