Adding GPU support to the GUI in Apple Silicon

[PikaPei]

Hello Cellpose team!

I noticed that M1 GPU support is in the command-line mode in #523, but not yet in GUI.

This PR checks if the platform is 'arm', the torch.device will use 'mps'.
I have tested it on my M2 macbook, and hope it's useful.

[sophiamaedler]

I tried installing cellpose from @PikaPei fork to see if I can get a GUI running with GPU support in Apple Silicon but I encountered the following error:

NET ERROR: Cannot convert a MPS Tensor to float64 dtype as the MPS framework doesn't support float64. Please use float32 instead.

Executing the same segmentation in a python script I got the following traceback:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/models.py", line 216, in eval
    diams, _ = self.sz.eval(x, channels=channels, channel_axis=channel_axis, invert=invert, batch_size=batch_size, 
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/models.py", line 889, in eval
    diam, diam_style = self.eval(x[i], 
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/models.py", line 925, in eval
    masks = self.cp.eval(x, 
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/models.py", line 552, in eval
    masks, styles, dP, cellprob, p = self._run_cp(x, 
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/models.py", line 652, in _run_cp
    outputs = dynamics.compute_masks(dP[:,i], cellprob[i], niter=niter, cellprob_threshold=cellprob_threshold,
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/dynamics.py", line 746, in compute_masks
    mask = remove_bad_flow_masks(mask, dP, threshold=flow_threshold, use_gpu=use_gpu, device=device)
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/dynamics.py", line 601, in remove_bad_flow_masks
    merrors, _ = metrics.flow_error(masks, flows, use_gpu, device)
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/metrics.py", line 282, in flow_error
    dP_masks = dynamics.masks_to_flows(maski, use_gpu=use_gpu, device=device)
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/dynamics.py", line 277, in masks_to_flows
    mu, mu_c = masks_to_flows_device(masks, device=device)
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/dynamics.py", line 152, in masks_to_flows_gpu
    mu = _extend_centers_gpu(neighbors, centers, isneighbor, Ly, Lx, 
  File "/Users/sophia/mambaforge/envs/cellpose2/lib/python3.9/site-packages/cellpose/dynamics.py", line 71, in _extend_centers_gpu
    T = torch.zeros((nimg,Ly,Lx), dtype=torch.double, device=device)
TypeError: Cannot convert a MPS Tensor to float64 dtype as the MPS framework doesn't support float64. Please use float32 instead.

By changing dtype=torch.double to dtype=torch.float32 in dynamics.py it seems to fix the issue I can successfully segment an image both in the GUI and in a python terminal using my GPU.

I am unfortunately not sure how to add this as an additional commit to this PR.