Fix ACER to support GaussianHeadWithFixedCovariance

[muupan]

Resolves #144

get_params_of_distribution now returns only parameters that require grad and raises an error if no parameter of a given distribution requires grad.

The loss function used in the test is changed because maximizing the entropy has no effect for Gaussian with fixed covariance.

[muupan]

/test

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[muupan]

/test

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