Commits on Mar 9, 2021

1. Remove memory scope and improve memory management …

   The MemoryScope reveals a number of shortcomings within the DJL memory
   management. While the MemoryScope is deleted, many of them are fixed as part of
   this PR.
   
   First, the NDManager.{attach, detach} were renamed to xxxInternal. This is to
   differentiate them from the attach and detach methods that are intended to be used.
   
   There are two new concepts in memory management. An NDResource interface was
   created to combine the concepts of managed memory that was used in NDArray and
   NDList. It could also be used in more classes in the future. This includes the
   getManager, attach, and detach.
   
   Within the NDManager, it gains a second "management convention". The first
   convention of normal resources are added to the manager and then closed when the
   manager closes. This works for small numbers of things on the NDArray, but not
   when operations transitively create. So, the second convention is a
   tempResource. Instead of freeing them when the manager is closed, they are
   returned to their original manager. This is used to create a temporary scope, do
   operations within it, and then the inputs and return value are returned to the
   parent while the intermediate work is cleaned. This also matches the concepts of
   ownership/borrowing as well.
   
   Using these, a few additional helper methods were created. There is
   `NDManager.from(resource)` to ease creation of managers based on a resource.
   There is also `scopeManager.ret(returnValue)` to help with returning values
   outside of the scopeManager. Lastly, there is a `scopeManager.{temp,}AttachAll`
   to attach a number of resources to a manager within a single call.
   
   Using these improvements, the new method were applied to the old locations where
   MemoryScope was used as well as an additional case in NDManagerEx.
   
   Also, the old attach methods were altered to be `void`. Because the return
   values are no longer used anywhere and are not as necessary in the current
   scheme, I figured it would simplify things. It also helps for things like
   `NDList.attach` which does not have a single original NDManager when attaching.
   
   Change-Id: I91d109cd14d70fa64fd8fffa0b50d88ab053013e

   

   zachgk committed Mar 9, 2021
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