Allow fake process groups to simulate any rank

[ezyang]

Fake mode always initialized global rank zero, which prevents exercising
MPMD code paths such as nonzero pipeline stages. Read RANK in fake mode
and validate it against the simulated world size while preserving rank
zero as the default.

[ezyang]

Stack from ghstack (oldest at bottom):

• Support fake pipeline runs one rank at a time #4019
• -> Allow fake process groups to simulate any rank #4018