Voxelize returns voxels outside of min/max bounds

**Describe the bug**
`open3d.ml`'s op `voxelize` returns voxels outside of the max/min bounds given to the function.

**To Reproduce**
1. Create point cloud with points outside of set bounds
2. Pass point cloud and bounds to `voxelize`
3. Voxel coordinates will correspond to locations outside of the bounds

An example of this bug in practice is https://github.com/isl-org/Open3D-ML/issues/349 which to fix requires downstream coordinate bounds checking/filtering https://github.com/isl-org/Open3D-ML/pull/350

**Expected behavior**
Voxels will not be generated with coordinates outside of the given bounds

**Environment (please complete the following information):**

- Operating system: Ubuntu 18.04
- Python version: Python 3.6.13
- Open3D version: 0.13.0
- Is this remote workstation?: no
- How did you install Open3D?: `pip`
- Compiler version (if built from source): N/A

**Additional context**
`voxelize` should filter points outside of the given bounds before performing voxelization