pushpull: fix push_backward passing the wrong stride array to the kernel

pushpull.h

@@ -531,10 +531,17 @@ void push_backward(
  offset_t inp_offset,
  offset_t ginp_offset)
  {
+ // The kernel's `stride` argument indexes `ginp` -- the *splatted
+ // volume* (shape `size_splinc`), which push_backward gathers from --
+ // so it must be `stride_ginp`, not `stride_inp` (which strides the
+ // grid-shaped forward input, and is only used for the `isc` channel
+ // step + the outer `inp_offset`). jitfields passed `stride_inp` here;
+ // that is latently wrong and only agrees when the pushed volume and
+ // the grid happen to have identical spatial strides.
  return PushPull<ndim, IX, BX, IY, BY, IZ, BZ, abs>::push_backward(
  out + out_offset, gout + gout_offset,
  inp + inp_offset, ginp + ginp_offset,
- loc, size_splinc + nbatch, stride_inp + nbatch,
+ loc, size_splinc + nbatch, stride_ginp + nbatch,
  nc, osc, isc, osg, isg, _bnd, _spl);
  };