Put `learn` back to FP32 precision mode.

def to_fp32(learn:Learner):
    "Put `learn` back to FP32 precision mode."
    learn.data.remove_tfm(batch_to_half)
    for cb in learn.callbacks: 
        if isinstance(cb, MixedPrecision): learn.callbacks.remove(cb)
    learn.model = learn.model.float()
    return learn