Train PFL with MLX

[grananqvist]

Support running PFL (including with DP) with MLX. Also includes the simple CNN benchmark.

Items that were left out of this initial integration and to be done on a case-by-case basis:

• Run other algorithms than FedAvg with MLX
• The remaining benchmarks (SO, FLAIR etc). I will add LM example in a follow-up PR to show that MLX is much faster than pytorch for non-CNN models.
• notebook tutorial (to be done in follow-up PR)