Pardons -

said that he has the “absolute right” to pardon himself of any and all crimes?

As has been stated by numerous legal scholars, I have the absolute right to PARDON myself, but why would I do that when I have done nothing wrong? In the meantime, the never ending Witch Hunt, led by 13 very Angry and Conflicted Democrats (& others) continues into the mid-terms!
— Donald J. Trump (@realDonaldTrump) June 4, 2018

The remarks were in reference to any potential charges stemming from the Special Council investigating Russian interference in US elections. While asserting his right to pardon himself, he denied any wrongdoing.

Article II, Section 2 of the Constitution explicitly states that the President “shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment.”

Time – President Trump Says He Can Pardon Himself. Most Voters Disagree
The Washington Post – Trump says he has ‘absolute right’ to pardon himself of federal crimes but denies any wrongdoing
Interactive Constitution – Article II, Executive Branch

considered pardoning Muhammed Ali, even though his conviction had already been overturned?

Ali’s conviction had already been overturned almost 50 years prior, so it is unclear what Trump sought to pardon. Ali’s family and estate attorney responded that while they “appreciate President Trump’s sentiment…a pardon is unnecessary.”

Muhammad Ali in 1978. *Dave Pickoff / Associated Press*

In 1971 the U.S. Supreme Court unanimously overturned Muhammad Ali’s conviction to refuse a draft into the Vietnam War. Ali’s lawyer remarked that “there is no conviction from which a pardon is needed.”

The New York Times – Trump Says He’s Considering a Pardon for Muhammad Ali