"Creator" seems fine, and originator ok, but I dislike the other two.
"Creator" seems appropriate. "X created Y" implies that prior to X's creative action Y did not exist and afterwards it did. Causing a theory to come into being may involve rearranging existing parts (mathematical operators, and paradigms that help understand formalisms at an intuitive level) and helping physicists understand it.
"Author" seems inappropriate. "X authored Y" implies that the significant part of the act was finding the right terms (broadly interpreted) to convey something. That was not the case here. The idea itself was novel.
"Originator" seems somewhat appropriate. "X originates from Y" implies that without Y there would be no X, but after the originating event X is no longer dependent on Y for continued existence. Many other words convey the former, but "originator" seems a poor choice unless you really want to convey the latter.
"Inventor" seems inappropriate. Inventors create inventions and "invention" is usually a physical device or process of some sort. It seems odd to use the word invention for a set of closed-form expressions and accompanying explanatory framework.
If what you wish to convey is that Einstein had an idea (a concept), did much of the hard work to formalize it, and presented it to the community of physicists, then maybe "conceiver."