Fix flax from_pretrained pytorch weoghts check

[mishig25]

The implementation was mostly copied from transformers.FlaxPreTrainedModel & I think there was an error (made PR huggingface/transformers#19133 as well)

cc: @younesbelkada github is not letting me add you as reviwer

from diffusers import FlaxUNet2DConditionModel

# should tell you to use `from_pt=True`
model, params = FlaxUNet2DConditionModel.from_pretrained("{PT_PATH}")

# works by converting the weights
model, params = FlaxUNet2DConditionModel.from_pretrained("{PT_PATH}", from_pt=True)

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[patil-suraj]

Good catch! Thanks for fixing this!