Having said that the segment I quoted is only the definition of child porn. The law prohibiting child porn [0, section c2] provides that:
"It shall be an affirmative defense to a charge of violating paragraph (1), (2), (3)(A), (4), or (5) of subsection (a) that— ...
the alleged child pornography was not produced using any actual minor or minors. No affirmative defense under subsection (c)(2) shall be available in any prosecution that involves child pornography as described in section 2256(8)(C)"
It is worth mentioning the segments of section A that are excluded from this defence:
Section 3B prohibts the advertisement/distribution/solicitation/etc of of material that is claimed to contain (i) "an obscene visual depiction of a minor engaging in sexually explicit conduct; or (ii) a visual depiction of an actual minor engaging in sexually explicit conduct;"
The relevant part of this is (i), where you would need to parse out the definition of "obscene" and "minor". Section 2256 defines minor as "any person under the age of eighteen years", however the courts would probably read it in this context in contrast to the phrase "actual minor". I could not fine the definition of "obscene" or "actual minor". Talk to a lawyer.
Section 6 relates to prohibits providing child porn to a minor.
Section 7 requires a depiction of an identifiable minor.
Moreover in the above quote it seems it could be considered "using" to use the visual identity of a minor.
I'd imagine that meant it you used any reference image of faces for your neural net there's a chance of violating the "letter" of this provision.
This of course if not legal advice.