Java Tensor hybrid, owns at::Tensor, no memcopy for java outputs. (#30501)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30501
**Motivation**:
In current state output of libtorch Module forward,runMethod is mem copied to java ByteBuffer, which is allocated, at least in some versions of android, on java heap. That could lead to intensive garbage collection.
**Change**:
Output java tensor becomes owner of output at::Tensor and holds it (as `pytorch_jni::TensorHybrid::tensor_` field) alive until java part is not destroyed by GC. For that org.pytorch.Tensor becomes 'Hybrid' class in fbjni naming and starts holding member field `HybridData mHybridData;`
If construction of it starts from java side - java constructors of subclasses (we need all the fields initialized, due to this `mHybridData` is not declared final, but works as final) call `this.mHybridData = super.initHybrid();` to initialize cpp part (`at::Tensor tensor_`).
If construction starts from cpp side - cpp side is initialiaed using provided at::Tensor with `makeCxxInstance(std::move(tensor))` and is passed to java method `org.pytorch.Tensor#nativeNewTensor` as parameter `HybridData hybridData`, which holds native pointer to cpp side.
In that case `initHybrid()` method is not called, but parallel set of ctors of subclasses are used, which stores `hybridData` in `mHybridData`.
Renaming:
`JTensor` -> `TensorHybrid`
Removed method:
`JTensor::newAtTensorFromJTensor(JTensor)` becomes trivial `TensorHybrid->cthis()->tensor()`
Test Plan: Imported from OSS
Differential Revision: D18893320
Pulled By: IvanKobzarev
fbshipit-source-id: df94775d2a010a1ad945b339101c89e2b79e0f83
