Flax: trickle down `norm_num_groups`

[pcuenca]

FlaxEncoder receives it here, but then it's ignored.

FlaxResnetBlock2D uses a hardcoded value.

Discovered by @younesbelkada in #589 (comment)

[patrickvonplaten]

Good point! Not sure how urgent at is but let's make sure we don't forget about it

[pcuenca]

Maybe appropriate for hacktoberfest?

[akash5474]

Hello @pcuenca and @patrickvonplaten , I would like to try to take on this issue if that's alright.

[patrickvonplaten]

Hey @akash5474,

This would be great!

[github-actions]

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