I can't find mention of him making that specific promise of pardoning all non-violent drug crimes.
I did find https://joebiden.com/justice/ saying:
> Biden believes no one should be in jail because of cannabis use. As president, he will decriminalize cannabis use and automatically expunge prior convictions. And, he will support the legalization of cannabis for medical purposes, leave decisions regarding legalization for recreational use up to the states, and reschedule cannabis as a schedule II drug so researchers can study its positive and negative impacts.
> Biden believes that no one should be imprisoned for the use of illegal drugs alone. Instead, Biden will require federal courts to divert these individuals to drug courts so they receive treatment to address their substance use disorder.
Quoting https://en.wikipedia.org/wiki/Expungement
> A very real distinction exists between an expungement and a pardon. When an expungement is granted, the person whose record is expunged may, for most purposes, treat the event as if it never occurred. A pardon (also called "executive clemency") does not "erase" the event; rather, it constitutes forgiveness. In the United States, an expungement can be granted only by a judge, while a pardon can be granted only by the President of the United States for federal offenses
I have no clue how Biden can promise to automatically expunge prior convictions if only judges can expunge a record.