Add a new variant of Adam optimizer called "PAdam"
[Please delete this text and describe your change here.
For bugfixes, please detail the bug and include a test case which your patch fixes.
If you are adding a new feature, please clearly describe the design, its rationale, the possible alternatives considered.
It is easiest to merge new features when there is clear precedent in other systems; we need to know we're taking
the right direction since it can be hard to change later.]
Add a new variant of Adam optimizer called "PAdam"
[Please delete this text and describe your change here. For bugfixes, please detail the bug and include a test case which your patch fixes. If you are adding a new feature, please clearly describe the design, its rationale, the possible alternatives considered. It is easiest to merge new features when there is clear precedent in other systems; we need to know we're taking the right direction since it can be hard to change later.]
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