Incorrect use of partial in function TweedieDistribution._rowwise_gradient_hessian

[bili2002]

Description

There is an error in the code for TweedieDistribution._rowwise_gradient_hessian when self.power = 3. A p parameter is passed to partial, however p is not defined in the function signature for inv_gaussian_log_rowwise_gradient_hessian.

[lbittarello]

Thanks, @bili2002! Could you fix it and extend the tests to include the inverse Gaussian case?