While I appreciate the sentiment and certainly what Assange has done, to my knowledge he is not accused of breaking any US law.
Ergo, there is no a US president can pardon.
The pardon would have to come from where ever the rape charge (which appears fake) came from.
Criminal espionage investigations are supervised by the FISA court. These are not made public. I would be shocked if there were not one against Assange.
(Except of course for Hillary’s violations of the espionage law, which are conducted corruptly by political allies within the Justice Department.)