class_docstr: torch.jit.script optimised Swish w/ memory-efficient checkpoint
  Inspired by conversation btw Jeremy Howard & Adam Pazske
  https://twitter.com/jeremyphoward/status/1188251041835315200
code: def backward(ctx, grad_output):
      x = ctx.saved_tensors[0]
      return swish_jit_bwd(x, grad_output)
  
imports: import torch
  from torch import nn
  from torch.nn import functional
label_desc: Use image node_id 3 for calling the SwishJitAutoFn obj's underlying member method code with example usage: obj.backward(ctx, grad_output) and returns: swish_jit_bwd