Fix serialization memory lifetime issue. (#30603)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30603
Pickler object needs to be kept in scope until data is written out to the
final serialized string. tensorData in particular is a reference to memory
owned by the descoped Pickle object.
Noticed this by inspection. In practice, this potential read-after-free here
is limited to non-cpu tensors, and any such use was very soon after free.
ghstack-source-id: 94756036
Test Plan: existing test suite at buck test mode/dev-nosan caffe2/test:rpc_fork
Differential Revision: D18760463
fbshipit-source-id: 9de890d66626aa48f13ca376dd9bd50b92e0cb00
