How to align caption to the left of a figure?

You can load the caption package and choose the option singlelinecheck=off, and the measuredfigure environment from threeparttable:

documentclass[conference]{IEEEtran}
usepackage[utf8]{inputenc} %
usepackage{graphicx, caption} %
usepackage{threeparttable}%
usepackage{lipsum}

begin{document}

lipsum[1]
begin{figure}[!htb]
  captionsetup{singlelinecheck = false, format= hang, justification=raggedright, font=footnotesize, labelsep=space}
  centering
  begin{measuredfigure} % begin{measuredfigure}
    includegraphics{Pepe-le-Pew2}
    caption{Chuck Jones – Pepe Le Pew in action}
  end{measuredfigure}
  label{PlP}
end{figure}
lipsum[2]

end{document} 

As suggested in this answer on the LaTeX Community forum (LaTeX.org), it's quite easy to achieve the left alignment by loading the package caption with the options singlelinecheck=false and justification=justified:

usepackage[singlelinecheck=false,justification=justified]{caption}

If you're interested in the details, you can read more about this in the caption package documentation in Sections 2.2 (pp. 9-10).