[PyTorch] IValue::toTensor can now return const Tensor&

[swolchok]

Stack from ghstack:

• [PyTorch] List::operator[] can return const ref for Tensor & string #50083 [PyTorch] List::operator[] can return const ref for Tensor & string
• [PyTorch] IValue::toTensor can now return const Tensor& #48868 [PyTorch] IValue::toTensor can now return const Tensor&
• [PyTorch] Store Tensor explicitly in IValue #48824 [PyTorch] Store Tensor explicitly in IValue
• [PyTorch] Additional IValue tests #49718 [PyTorch] Additional IValue tests

Building on the previous diff, we can make toTensor() return a
const Tensor&, which should make it easier to avoid reference
counting.

Differential Revision: D25325379

NOTE FOR REVIEWERS: This PR has internal Facebook specific changes or comments, please review them on Phabricator!

[dr-ci]

💊 CI failures summary and remediations

As of commit 1325e5d (more details on the Dr. CI page):

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• 2/2 failures possibly* introduced in this PR
  • 1/2 non-CircleCI failure(s)

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🕵️ 1 new failure recognized by patterns

The following CI failures do not appear to be due to upstream breakages:

pytorch_linux_bionic_py3_8_gcc9_coverage_test1 (1/1)

Step: "Run tests" (full log | diagnosis details | 🔁 rerun)

Jan 05 01:22:24 [E request_callback_no_python.cpp:636] Received error while processing request type 258: RuntimeError: Can not pickle torch.futures.Future

Jan 05 01:22:24 At:
Jan 05 01:22:24   /opt/conda/lib/python3.8/site-packages/torch/distributed/rpc/internal.py(120): serialize
Jan 05 01:22:24   /opt/conda/lib/python3.8/site-packages/torch/distributed/rpc/internal.py(172): serialize
Jan 05 01:22:24 
Jan 05 01:22:24 [E request_callback_no_python.cpp:636] Received error while processing request type 258: RuntimeError: Can not pickle torch.futures.Future
Jan 05 01:22:24 
Jan 05 01:22:24 At:
Jan 05 01:22:24   /opt/conda/lib/python3.8/site-packages/torch/distributed/rpc/internal.py(120): serialize
Jan 05 01:22:24   /opt/conda/lib/python3.8/site-packages/torch/distributed/rpc/internal.py(172): serialize
Jan 05 01:22:24 
Jan 05 01:22:24 [E request_callback_no_python.cpp:636] Received error while processing request type 258: RuntimeError: Can not pickle torch.futures.Future
Jan 05 01:22:24 
Jan 05 01:22:24 At:
Jan 05 01:22:24   /opt/conda/lib/python3.8/site-packages/torch/distributed/rpc/internal.py(120): serialize
Jan 05 01:22:24   /opt/conda/lib/python3.8/site-packages/torch/distributed/rpc/internal.py(172): serialize
Jan 05 01:22:24 
Jan 05 01:22:24 [W tensorpipe_agent.cpp:547] RPC agent for worker0 encountered error when reading incoming request from worker3: EOF: end of file (this is expected to happen during shutdown)
Jan 05 01:22:24 [W tensorpipe_agent.cpp:547] RPC agent for worker0 encountered error when reading incoming request from worker2: EOF: end of file (this is expected to happen during shutdown)
Jan 05 01:22:25 ok (2.551s)
Jan 05 01:22:27   test_return_future_remote (__main__.TensorPipeRpcTestWithSpawn) ... [W tensorpipe_agent.cpp:547] RPC agent for worker1 encountered error when reading incoming request from worker0: EOF: end of file (this is expected to happen during shutdown)
Jan 05 01:22:27 ok (2.554s)

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1 job timed out:

• pytorch_linux_bionic_py3_8_gcc9_coverage_test1

---

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This pull request has been merged in a440491.