title: Why should be the function backward be called only on 1 element tensor or with gradients w.r.t to Variable?
body: <p>I am new to pytorch. I want to understand as to why we can't call the backward function on a variable containing a tensor of say size say [2,2].
  And if we do want to call it on a variable containing tensor of say size say [2,2], we have to do that by first defining a gradient tensor and then calling the backward function on the variable containing the tensor w.r.t the defined gradients.</p>