Add get_config and from_config methods to DenseVariational_v2

[Frightera]

Fixes the issue #1627:
NotImplementedError: Layer DenseVariational has arguments in __init__ and therefore must override get_config

Some unit tests are failing due to an internal issue in TF-Nightly as of 17/02/2023. Please find the gist here to see the fix.

Thanks!

[Frightera]

Hi @csuter, is there any chance that you can take a look at this tiny PR?

Thanks.

[csuter]

thanks for sending.

please add some unit tests covering the new behavior, in the corresponding test file.

would any other layers benefit from adding the same new behavior?