How to align comment in algorithm code?

You can use the \makebox[width][position]{text} command to create a box of a specified width, allowing you to control the alignment of the text within it.

\documentclass{article}
\usepackage[a4paper]{geometry}

\usepackage{algorithm, algcompatible, algpseudocode}
\usepackage{xcolor}% needed for the color 'blue'

\newcommand{\Bar}[1]{\bar{#1}}

% 0. this seems to be the package definition
% \renewcommand{\algorithmiccomment}[1]{\hfill\(\triangleright\) #1}

% 1. let's align all comments to the right within a 7cm box
% \renewcommand{\algorithmiccomment}[1]{\hfill\makebox[7cm][l]{\(\triangleright\) #1}}
% 2. let's use \fbox to visually check the box limits
% \renewcommand{\algorithmiccomment}[1]{\hfill\fbox{\makebox[7cm][l]{\(\triangleright\) #1}}}

% 3. Bonus tip: let's make the comments stand out with different font and color
\newcommand\commentFont[1]{\small\ttfamily\textcolor{blue}{#1}}
\renewcommand{\algorithmiccomment}[1]{\hfill\makebox[7cm][l]{\commentFont{// #1}}}

\begin{document}
\begin{algorithm}\caption{Steps of computation in a full GN block}\label{alg:gn_block}
    \begin{algorithmic}[1]
    \Function{GraphNetwork}{$E, V, u$}
        \For{$k \in \{1 \dots N_e\}$}
            \State $e_k' \gets \phi^e(e_k, E_{r_k}, E_{s_k}, u)$ \Comment{1. Compute updated edge attributes}
        \EndFor
        \For{$i \in \{1 \dots N_v\}$}
            \State $E_i' \gets \{(e_k', r_k, s_k)\}_{r_k = i, k = 1:N_e}$
            \State $\Bar{e}_i' \gets \rho^{e \to v}(E_i')$ \Comment{2. Aggregate edge attributes per node}
            \State $E_i' \gets \phi^v(\Bar{e}_i', E_i, u)$ \Comment{3. Compute updated node attributes}
        \EndFor
        \State $E' \gets \{E_i'\}_{i = 1:N_v}$
        \State $E' \gets \{(e_k', r_k, s_k)\}_{k = 1:N_e}$
        \State $\Bar{e}' \gets \rho^{e \to u}(E')$ \Comment{4. Aggregate edge attributes globally}
        \State $\Bar{v}' \gets \rho^{v \to u}(E')$ \Comment{5. Aggregate node attributes globally}
        \State $u' \gets \phi^u(\Bar{e}', \Bar{v}', u)$ \Comment{6. Compute updated global attribute}
        \State \Return $(E', V', u')$
    \EndFunction
    \end{algorithmic}
\end{algorithm}
\end{document}

You can set all the elements inside a box of equal width. This can be automated using eqparbox's \eqmakebox[<tag>][<align>]{<stuff>} that sets all <stuff> inside a box of maximum width across the same <tag>. An additional <align>ment can be specified.

The code below automatically defined the <tag> to be algorithm-dependent, so you can use it across different algorithms.

\documentclass{article}

\usepackage{algorithm,algpseudocode}
\usepackage{amsmath}
\usepackage{eqparbox}

\NewCommandCopy\OldComment\Comment
% Update \Comment to utilize \eqmakebox[<tag>][<align>]{<stuff>}
\RenewDocumentCommand{\Comment}{m}{\OldComment{\eqmakebox[alg-\thealgorithm][l]{#1}}}

\begin{document}

\begin{algorithm}
  \caption{Steps of computation in a full GN block}
  \begin{algorithmic}[1]
    \Function{GraphNetwork}{$E, V, u$}
      \For{$k \in \{1, \dots, N_e\}$}
        \State $e_k' \gets \phi^e(e_k, E_{r_k}, E_{s_k}, u)$
          \Comment{1. Compute updated edge attributes}
      \EndFor
      \For{$i \in \{1, \dots, N_v\}$}
        \State $E_i' \gets \{(e_k', r_k, s_k)\}_{r_k = i, k = 1:N_e}$
        \State $\Bar{e}_i' \gets \rho^{e \to v}(E_i')$
          \Comment{2. Aggregate edge attributes per node}
        \State $E_i' \gets \phi^v(\Bar{e}_i', E_i, u)$
          \Comment{3. Compute updated node attributes}
      \EndFor
      \State $E' \gets \{E_i'\}_{i = 1:N_v}$
      \State $E' \gets \{(e_k', r_k, s_k)\}_{k = 1:N_e}$
      \State $\Bar{e}' \gets \rho^{e \to u}(E')$
        \Comment{4. Aggregate edge attributes globally}
      \State $\Bar{v}' \gets \rho^{v \to u}(E')$
        \Comment{5. Aggregate node attributes globally}
      \State $u' \gets \phi^u(\Bar{e}', \Bar{v}', u)$
        \Comment{6. Compute updated global attribute}
      \State \Return $(E', V', u')$
    \EndFunction
  \end{algorithmic}
\end{algorithm}

\end{document}

Since \eqmakebox uses a \label-\ref-like approach to finding the widest element across the same <tag>, two compilations are needed with every change in the widest element (including the first compilation).