fix: is_cuda_tensor guarded by TORCH_AVAILABLE. #1522

Job	Run time
get-commit-message	4s
test-cpu (finufft)	16m 36s
test-cpu (pynfft)	4m 31s
test-cpu (sigpy)	4m 35s
test-gpu (cufinufft)	12m 57s
test-gpu (gpunufft)	12m 12s
test-gpu (torchkbnufft-gpu)	14m 28s
test-examples	23m 32s
Build API Documentation	10m 3s
coverage	28s
Compile the coverage badge in docs	20s
	1h 39m 46s

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