fix(optim/meta): torch tensor memory not release due to gradient link

[ycsos]

Description

when use torchopt.MetaAdam and step some times, the memory use in gpu are continuously increase. It should not be, will you excute next step, the tensor create in the former step is no need should be release. I find the reason: metaOptimizer not detach the gradient link in optimizer. and former tensor was not release by torch due to dependency.

you can run the test code, the first one memory increase by step increase. and second one (I change the code to detach the grad link) the memory is stable when step increase:

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