I'd like to choose a scientific paper template in LaTeX for a Deep Learning (Artificial Intelligence) project. I think the AAAI or the IEEE are interesting, however, I'd like to know under which terms I can use their templates if my work is not intended for publication. (I am also open to suggestions of other templates or styles to consider.)
Virtually all templates available as packages on the Comprehensive TeX Archive Network (CTAN) are free to use, regardless of whether or not they're intended for submission to the journal for which they were created. If there are any specific restrictions for their use, they would be indicated somewhere in the package information.
While this particular project may remain unpublished, are you planning on publishing in the future? If you think you might, then it would probably be worth your time to use the template of the conference to which you plan to submit. Each template can have its own quirks, and so it is worth getting used to these.
Additionally, while you think this work may end up being unpublished, there is nothing preventing the future you from turning the work into a publication in the future. AAAI and IEEE aren't going to prevent you from using their templates for draft papers, so again, I'd suggest using the template for the venue you are most likely to publish in in the future.
Another point of consideration is where you are currently reading most of your papers. If you are reading mostly AAAI papers, then it is good experience writing your own papers in this style to see how your paper compares to those that are published and accepted in the area/conference/venue.
Finally, take a look at the preamble to the AAAI 2018 style file. You can use it for your own purposes as long as your follow their guidelines. (Put simply, if you modify the file, remove aaai from the name, and make sure you keep the acknowledgements.)
% WARNING: IF YOU ARE USING THIS STYLE SHEET FOR AN AAAI PUBLICATION, YOU % MAY NOT MODIFY IT FOR ANY REASON. MODIFICATIONS (IN YOUR SOURCE % OR IN THIS STYLE SHEET WILL RESULT IN REJECTION OF YOUR PAPER). % % NOTICE: DO NOT MODIFY THIS FILE WITHOUT CHANGING ITS NAME. % This style file is called aaai18.sty. Modifications to this file are permitted, % provided that your modified version does not include the acronym "aaai" % in its name, that credit to the authors and supporting agencies is % retained, and that further modification or reuse is not restricted. This % file was originally prepared by Peter F. Patel-Schneider, liberally % using the ideas of other style hackers, including Barbara Beeton. It was % modified in April 1999 by J. Scott Penberthy and George Ferguson. It was % modified in February 2009 and in November 2009 by Hans W. Guesgen and % Giuseppe De Giacomo. It has been modified annually by AAAI since 2010. It was last % modified on 18 July 2017 for the AAAI-18 conference. % The original preparation of this file was supported by % Schlumberger Palo Alto Research, AT\&T Bell Laboratories, AAAI, and % Morgan Kaufmann Publishers. The current version was supported by the % Association for the Advancement of Artificial Intelligence. % % WARNING: This style is NOT guaranteed to work. It is provided in the % hope that it might make the preparation of papers easier, but this style % file is provided "as is" without warranty of any kind, either express or % implied, including but not limited to the implied warranties of % merchantability, fitness for a particular purpose, or noninfringement. % You use this style file at your own risk. Standard disclaimers apply. % There are undoubtably bugs in this style. If you would like to submit % bug fixes, improvements, etc. please let us know. Please use the contact form % at www.aaai.org.
(The AAAI 2018 author kit which contains this file is currently available here.)