Added options to run mofa in SVI mode

[sagar87]

Added some arguments that enable to run MOFA using SVI. Made my life a bit easier, though I didn't test it thoroughly. Thanks for the great package.

[gtca]

Hey @sagar87, thanks for this PR!
As the interface for MOFA has grown quite extensive already, I added svi_ prefix to the new arguments to semantically group them. I have also changed the defaults to the ones used in the canonical MOFA implementation in R.

Hope that makes sense. Feel free to open new issues or PRs if you think there's anything else.