How to keep the subsection counter when using `part' in Beamer?

I’m using Beamer to create my lecture notes. I’m using `part{}’ to separate individual lectures. However, I find out that if a part starts with a subsection, then the subsection counter is automatically reset. How can I avoid this? Thanks!

Here is a minimal running example. The subsection counter in the second part is reset as 1, which is supposed to be 3.

useoutertheme[width = 1.5cm, height = 1cm]{sidebar}
begin{document}


%=================== Lecture 1 ======================
part{}
section{section 1}
subsection{subsection 1.1}
subsection{subsection 1.2}
section{section 2}
subsection{subsection 2.1}
subsection{subsection 2.2}
begin{frame}{Page 1}
     The section counter in Part thepart is thesection.
     The subsection counter in Part thepart is thesubsection.
end{frame}


part{}
subsection{subsection 2.3}
begin{frame}{Page 2}
    The section counter in Part thepart is thesection.
     The subsection counter in Part thepart is thesubsection.
end{frame}
end{document}

documentclass{beamer}
usetheme{Hannover}
setbeamertemplate{section in toc}[sections numbered]
setbeamertemplate{subsection in toc}[subsections numbered]
setbeamertemplate{subsubsection in toc}[subsubsections numbered]
setcounter{secnumdepth}{3}

begin{document}
begin{frame}{Titre}
tableofcontents
end{frame}
section{Rappels}
subsection{Fondements}
begin{frame}{Titre}
Bla bla.
end{frame}

begin{frame}{Titre}
Bla bla.
end{frame}

begin{frame}{Titre}
Bla bla.
end{frame}
subsection{Implications}
begin{frame}{Titre}
Bla bla.
end{frame}
subsection{Critiques}
begin{frame}{Titre}
Bla bla.
end{frame}
section{Th'eorie}
subsection{Plan}
begin{frame}{Titre}
Bla bla.
end{frame}
subsection{'Equilibre}
begin{frame}{Titre}
Bla bla.
end{frame}
end{document}
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