Return a dropout mask of the same type as `x`, size `sz`, with probability `p` to cancel an element.

def dropout_mask(x:Tensor, sz:Collection[int], p:float):
    "Return a dropout mask of the same type as `x`, size `sz`, with probability `p` to cancel an element."
    return x.new(*sz).bernoulli_(1-p).div_(1-p)