Modify the order of BN and Conv of RepMixer

[dheerajvarma24]

FastViT paper has the BN -> Conv order in the Repmixer block. But in this repo, it is in the reverse order. Included a flag 'use_bn_conv' to achieve this order, only for the RepMixer Block. I attached the screenshots of the model before and after the change.

RepMixer_block_FastViT_code Corrected_RepMixer_block

@anuragranj @pavank-apple please have a look at this change.