Removed unused imports in `jax.experimental.mosaic.gpu.core` #62555

Job	Run time
build with 3.10 (py 3.10 on ubuntu-20.04-16core, x64=1)	7m 41s
Documentation - test code snippets (3.10)	1m 29s
build with 3.12 (py 3.12 on ubuntu-20.04-16core, x64=0)	6m 1s
Documentation - render documentation (3.10)	6m 39s
FFI example	49s
jax2tf_test (py 3.10 on ubuntu-latest, x64=0)	1m 11s
lint_and_typecheck	1m 5s
	24m 55s

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