Recursively map lists of tensors in `b ` to FP16.

def to_float(b:Collection[Tensor])->Collection[Tensor]:
    "Recursively map lists of tensors in `b ` to FP16."
    if is_listy(b): return [to_float(o) for o in b]
    return b.float() if b.dtype not in [torch.int64, torch.int32, torch.int16] else b