since your question seems to be unstable system ... i'm afraid that my answer will miss your point as it was Mico answer (which finish with deletion) :-( )
anyway, for exercise:
- your rotated text is taller than rest of table, so it can not bo properly centered
- easier way to rotate cell content is use
\rothead
macro from makecell
package (for which you need to define cell height)
- vertically centering in
multirow
cell needs some manual tweaking
with this (and expand the table) I obtain the following result:

MWE:
\documentclass{article}
\usepackage{rotating}
\usepackage{array,makecell,multirow}
\begin{document}
\begin{table}[htb]
\centering
\settowidth\rotheadsize{Supervised Learning}
\begin{tabular}{| l | c |
>{\centering\arraybackslash}p{8cm}|}
\hline
\multicolumn{2}{|c|}{Classification} & References \\ \hline
\multirow{8}{*}[1.5ex]{\rothead{Supervised Learning}}
& & 1 \\ \cline{2-3}
& & 2 \\ \cline{2-3}
& & 3 \\ \cline{2-3}
& & 4 \\ \cline{2-3}
& & 5 \\ \cline{2-3}
& & 6 \\ \cline{2-3}
& & 7 \\ \cline{2-3}
& & 8 \\ \hline \end{tabular}
\caption{A Classification}
\label{tab:Machine_Learning_models}
\end{table}
\end{document}
Addendum:
Font size of text in \rothead
is determined by \headfont
macro. Default value is \footnotesize
. It can be changed by
\renewcommand\theadfont{\normalsize}
(or whatever size, font family and shape you like). Considering above change of font size, the relvant part of MWE become:
\documentclass{article}
\usepackage{rotating}
\usepackage{array,makecell,multirow}
\renewcommand\theadfont{\normalsize}% <-- added
\begin{document}
\begin{table}[htb]
\centering
\settowidth\rotheadsize{\theadfont Supervised Learning}% <-- changed
\begin{tabular}{| l | c |
>{\centering\arraybackslash}p{8cm}|}
\hline
\multicolumn{2}{|c|}{Classification} & References \\ \hline
\multirow{8}{*}{\rothead{Supervised Learning}}% <-- changed
% further is the same as in above MWE
and table will looks as follows:

rotating
– Amir Aug 28 '16 at 19:16