Returns the value of the deviance function.
Parameters: | endog : array-like
mu : array-like
freq_weights : array-like
scale : float, optional
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Returns: | deviance : float
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Notes
D = \sum_i piecewise_i where piecewise_i is defined as:
If Y_{i} = 0:
piecewise_i = 2* \log(1 + \alpha * \mu_i) / \alpha
If Y_{i} > 0: