Fix density update for gpuNUFFT #1303

Job	Run time
get-commit-message	4s
get-commit-message	4s
get-commit-message	4s
test-cpu (finufft)	13m 33s
test-cpu (finufft)	13m 33s
test-cpu (finufft)	13m 33s
test-gpu (cufinufft)	38s
test-gpu (cufinufft)	36s
test-gpu (cufinufft)	3s
test-cpu (pynfft)	5m 24s
test-cpu (pynfft)	5m 24s
test-cpu (pynfft)	5m 24s
test-gpu (gpunufft)	40s
test-gpu (gpunufft)	36s
test-gpu (gpunufft)	6m 50s
test-cpu (sigpy)	5m 35s
test-cpu (sigpy)	5m 35s
test-cpu (sigpy)	5m 35s
test-gpu (torchkbnufft-gpu)	2s
test-gpu (torchkbnufft-gpu)	4s
test-gpu (torchkbnufft-gpu)	1s
test-examples	19m 27s
test-examples	19m 27s
test-examples	19m 27s
Build API Documentation	0s
Build API Documentation	0s
Build API Documentation	0s
coverage	24s
coverage	29s
coverage	23s
Compile the coverage badge in docs	0s
Compile the coverage badge in docs	0s
Compile the coverage badge in docs	0s
	2h 22m 55s

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