Add tutorial on memory management

See Issue https://github.com/Project-MONAI/MONAI/issues/6626 relating to memory management and interaction with the garbage collector. One solution that we've tried before and had success with was to use the GarbageCollector handler to clean unused references at certain times. This isn't ideal and not particularly Pythonic but without changes to the garbage collector to consider GPU memory there may be little choice. Certain design decisions and good practices can reduce the issue so a tutorial on the problem and how to deal with OOM issues would be really helpful.

Start by reading linked issue #6626 and reviewing the GarbageCollector handler approach described here. Create a tutorial covering the memory-management problem, design practices that reduce out-of-memory issues, and how the handler can help clean unused references. Done means the tutorial clearly explains the problem and practical ways to address it.