Summarize the bug (e.g., "Segmentation Fault for Colored Point Cloud Registration")

[wangmiaowei]

Checklist

• [X] I have searched for similar issues.
• [X] For Python issues, I have tested with the latest development wheel.
• [X] I have checked the release documentation and the latest documentation (for master branch).

Describe the issue

I have been running PCD.estimate_normals both on GPU and CPU but the speed on GPU is really slow!

Steps to reproduce the bug

import time
start = time.time()
pcd.estimate_normals()
end =time.time()
print('cpu process time: ',end-start)

pcd_cuda = pcd.cuda()
start = time.time()
pcd_cuda.estimate_normals()
end =time.time()
print('cpu process time: ',end-start)

Error message

No response

Expected behavior

No response

Open3D, Python and System information

- Operating system: Ubuntu 20.04 / macOS 10.15 / Windows 10 64-bit
- Python version: Python 3.8 / output from `import sys; print(sys.version)`
- Open3D version: output from python: `print(open3d.__version__)`
- System architecture: x86 / arm64 / apple-silicon / jetson / rpi
- Is this a remote workstation?: yes or no
- How did you install Open3D?: pip / conda / build from source
- Compiler version (if built from source): gcc 7.5 / clang 7.0

Additional information

No response