Evaluates the MLE of the Poisson distribtion.
The PMF of the Poisson distribution is as follows:
$$
f\left(X=x\mid\,\theta\right)=\frac{\theta^{x}e^{-\theta}}{x!};\qquad x=0,1,2,\dots,
$$
where parameter \(\theta>0\) and it is equal to the expected or mean value of X and also to its variance.