[doc] fix `torch.cuda.mem_get_info` doc (#96621)
the current `torch.cuda.mem_get_info` doc is incorrect. This util returns `free, total` and not `free, used`
```
__host__ ​cudaError_t cudaMemGetInfo ( size_t* free, size_t* total )
Gets free and total device memory.
```
Also this util isn't mentioned in https://pytorch.org/docs/stable/notes/cuda.html#cuda-memory-management - should it be included there as well?
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96621
Approved by: https://github.com/kit1980
