[loading] Re-add and improve disk offloading support (#42242)
* unskip tests
* first shot
* offload in safetensors format
* remove hard-coded value
* update error
* typo
* fix
* update test
* fix
* return it
* post rebase
* improve var names
* improve names
* fix finally
* comment
* fix tests
* fix
* simplify
* fix
* doc
* fix
* remove additional tiying after rebase
* update test function source
* fix
* post rebase
* new renaming patterns
* clear confusion about variable names
* create cleaner function
* better doc
* other tests
* remove skip
* unskip other tests