Fix Tensor::register_hook behavior on undefined tensors (#105587)
When the hook registered by Tensor::register_hook (in C++) gets passed
an undefined tensor, it raises an internal assert in debug mode.
The cause is that we attempt to construct an OptionalTensorRef
(https://github.com/pytorch/pytorch/blob/4448c78a5d8ce91573fb8779355838bc78bfea33/aten/src/ATen/core/Tensor.h#L68)
which asserts that the passed-in TensorBase is defined.
The fix is that we create a new TensorRef class to convert the
TensorBase into a Tensor without bumping the refcount (which is what
OptionalTensorRef does). We cannot reuse OptionalTensorRef because
OptionalTensorRef represents `optional<Tensor>` that cannot hold an
Undefined Tensor.
For some more historical context, it looks like this behavior was introduced
in #63612
Test Plan:
- new tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105587
Approved by: https://github.com/soulitzer
