Add warning when accessing Tensor::grad() in the C++ API (#59362)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/35379
- Adds `retains_grad` attribute backed by cpp as a native function. The python bindings for the function are skipped to be consistent with `is_leaf`.
- Tried writing it without native function, but the jit test `test_tensor_properties` seems to require that it be a native function (or alternatively maybe it could also work if we manually add a prim implementation?).
- Python API now uses `retain_grad` implementation from cpp
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59362
Reviewed By: jbschlosser
Differential Revision: D28969298
Pulled By: soulitzer
fbshipit-source-id: 335f2be50b9fb870cd35dc72f7dadd6c8666cc02
