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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def call_forward(self, vol_data, out=None):
 """Run an ASTRA forward projection on the given data using the GPU. Parameters vol_data : ``reco_space`` element Volume data to which the projector is applied. out : ``proj_space`` element, optional Element of the projection space to which the result is written. If ``None``, an element in `proj_space` is created. Returns ------- out : ``proj_space`` element Projection data resulting from the application of the projector. If ``out`` was provided, the returned object is a reference to it. """

        with self._mutex:
            assert vol_data in self.reco_space
            if out is not None:
                assert out in self.proj_space
            else:
                out = self.proj_space.element()

            # Copy data to GPU memory
            if self.geometry.ndim == 2:
                astra.data2d.store(self.vol_id, vol_data.asarray())
            elif self.geometry.ndim == 3:
                astra.data3d.store(self.vol_id, vol_data.asarray())
            else:
                raise RuntimeError('unknown ndim')

            # Run algorithm
            astra.algorithm.run(self.algo_id)

            # Copy result to host
            if self.geometry.ndim == 2:
                out[:] = self.out_array
            elif self.geometry.ndim == 3:
                out[:] = np.swapaxes(self.out_array, 0, 1).reshape(
                    self.proj_space.shape)

            # Fix scaling to weight by pixel size
            if isinstance(self.geometry, Parallel2dGeometry):
                # parallel2d scales with pixel stride
                out *= 1 / float(self.geometry.det_partition.cell_volume)

            return out