fix loading flax bf16 weights in pt
# What does this PR do?

#13098 now enables saving flax weights in bf16. But converting flax bf16 weight to pt fails because `torch.from_numpy` can not handle `bfloat16` and `bfloat16` is also not fully supported by PT.

This PR fixes this by casting flax `bf16` weights to `fp32` when converting flax weights to PT.