The Custom validators are useful when you want to change a particular behavior of an existing grant. That is often needed because of the diversity of the identity software and to let the oauthlib framework to be flexible as possible.
However, if you are looking into writing a custom grant type, please refer to the Custom Grant Type </oauth2/grants/custom_grant> instead.
oauthlib.oauth2.rfc6749.grant_types.base.ValidatorsContainer