def requires_grad(m:nn.Module, b:Optional[bool]=None)->Optional[bool]:
    "If `b` is not set return `requires_grad` of first param, else set `requires_grad` on all params as `b`"
    ps = list(m.parameters())
    if not ps: return None
    if b is None: return ps[0].requires_grad
    for p in ps: p.requires_grad=b
If `b` is not set return `requires_grad` of first param, else set `requires_grad` on all params as `b`