\documentclass[../main.tex]{subfiles}

\makeatletter
\@ifundefined{fromRoot}{%
  \newcommand{\fromRoot}[1]{../#1}
  
  \usepackage{xr}
  \externaldocument{../main}
}{}

\def\input@path{{\subfix{../}}}
%or: \def\input@path{{/path/to/folder/}{/path/to/other/folder/}}
\makeatother

\graphicspath{{\subfix{../}}}

\hypersetup{
    pdfauthor   = {Author},
    pdftitle    = {Th\`{e}se (Conclusion)},
    pdfsubject  = {Th\`{e}se (Conclusion)},
%    pdfkeywords = {mots-cl\'{e}s},
}


\begin{document}

\selectlanguage{english}

\setmainfont{Latin Modern Roman}
\setmonofont{Latin Modern Math}

\chapter{Conclusion} \label{chapter:conclusion}
\chaptermark{Conclusion}

\section{Synthesis}

\section{Perspectives}


\begin{sidewaysfigure}
  \includegraphics[width=\linewidth]{figures/pgfplots/score_demo_raw_stdl.pdf}
  \caption{A sidewaysfigure}
\end{sidewaysfigure}

\end{document}