sigpy.mri.poisson hangs when used to generate 1d undersampling trajectories

**Describe the bug**
Is sigpy.mri.poisson designed to support 1D sampling pattern generation?
The following code snippet would hangs the function, without any error.
```
mask1 = sigpy.mri.poisson(
  (256,1),
  ...
)

mask2 = sigpy.mri.poisson(
  (1,256),
  ...
)
```
