Spacing transform outputs float64 Tensor data

[Nic-Ma]

Describe the bug
The PR #3045 introduced an issue that it converted the output data dytpe to the computation dtype, actually it should always output float32:
https://github.com/Project-MONAI/MONAI/blob/0.7.0/monai/transforms/spatial/array.py#L127
And I think no need to deepcopy the image:
https://github.com/Project-MONAI/MONAI/blob/dev/monai/transforms/spatial/array.py#L203