need an an easier way to activate memory usage debug

Currently when there are OOM problems we are being asked to modify deepspeed code to enable see_memory_usage - but it's not trivial - once needs to know where to enable as it's not just one place. And it's an issue in a shared env, unless one creates a separate env to debug.

I suggest adding a new debug feature that can change the behavior of the project from outside w/o needing to modify the source code, that is using env vars to do that. It shouldn't impact performance as it could assign noop functions at startup time.

So I propose to create an env var, e.g.: DS_MEM_SEE_USAGE=stage3,partitioning,engine,etc so really it's about defining the granularity of the areas, remapping those to the corresponding names, and then implementing this in the code.

Thanks.

No files or tests are named. Start by locating the existing see_memory_usage calls in the stage3, partitioning, and engine areas, then determine how an environment variable could select their granularity. Done means memory-usage debugging can be enabled externally without source edits and disabled with no performance impact.