* fixes memory leak when paramter is in fp16 in ZeroDDP init.
* bans chunk releasement in CUDA. Only when a chunk is about to offload, it is allowed to release.
* adds a constant placement policy. With it, users can allocate a reserved caching memory space for parameters.
* add placement policy
* add gemini mgr
* update mem stats collector
* update zero
* update zero optim
* fix bugs
* zero optim monitor os
* polish unit test
* polish unit test
* add assert
* polish chunk manager
* polish unit test
* impl add_extern_static_tensor for chunk mgr
* add mem stats collector v2
* polish code
* polish unit test
* polish code
* polish get chunks
* add zero optimizer
* torch ok
* unit test ok
* polish code
* fix bugs
* polish unit test
* polish zero optim
* polish colo ddp v2
* refactor folder structure
* add comment
* polish unit test
* polish zero optim
* polish unit test