documentclass[preview, varwidth=19.05cm]{standalone}

usepackage[utf8]{inputenc}
usepackage{amsmath, amssymb}
usepackage{graphicx}
usepackage{subcaption}

% Matplotlib2TikZ
usepackage{pgfplots}
pgfplotsset{compat=1.14}

begin{document}
begin{figure}
    captionsetup[subfigure]{justification=centering}
    centering

    begin{subfigure}[t]{8.8cm}
        centering
        input{Fig2a.tex}label{Fig2a}
        caption{Task A: Ground truth}
    end{subfigure}
    ~
    begin{subfigure}[t]{8.8cm}
        centering
        input{Fig2b.tex}
        caption{Task B: Ground truth}
        label{Fig2b}
    end{subfigure}

    begin{subfigure}[t]{8.8cm}
        input{Fig2c.tex}
        vspace{-0.5em}
        caption{Task A: Real-time annotations}
        label{Fig2c}
    end{subfigure}
    ~
    begin{subfigure}[t]{8.8cm}
        input{Fig2d.tex}
        vspace{-0.5em}
        caption{Task B: Real-time annotations}
        label{Fig2d}
    end{subfigure}
end{figure}
end{document}