apex.amp was developed as the first implementation for mixed-precision training before @mcarilli implemented the native implementation.
The main advantage is that you can use it by installing the (nightly) binaries without building apex as an extension (which is not always straightforward, if it’s your first time). In the long run we’ll move our focus on native amp and deprecate apex.amp.
Here is also a more detailed description.
It’s hard to tell, which of these changes created the OOM issue suddently without seeing the code.