The Form I-9 is required for individuals who are hired for employment in the United States, regardless of their citizenship or immigration status. The purpose of the form is to verify the identity and employment authorization of individuals working in the U.S.
If an individual is attending training in the United States but will not be working or providing services in the U.S., they may not be considered an "employee" for the purposes of Form I-9 requirements. In this case, it may not be necessary for them to complete the form.
However, if the individual will be providing any services or labor in the United States as part of their training or if they are considered an employee while attending training, then they would need to complete a Form I-9.
It is important to consult with legal counsel or human resources professionals to determine the specific circumstances and requirements for your situation.