[pytree] Only import optree if it's used

[zou3519]

Stack from ghstack (oldest at bottom):

• -> [pytree] Only import optree if it's used #131478

torch.utils._pytree imports optree if it's available. Instead, we change
it to if it gets used. The motivation for this is better isolation.

Test Plan:

• new tests

[pytorch-bot]

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✅ You can merge normally! (2 Unrelated Failures)

As of commit bdef50d with merge base 406f510 ():

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• trunk / linux-focal-cuda12.4-py3.10-gcc9-experimental-split-build-test / test (default, 1, 5, linux.4xlarge.nvidia.gpu) (gh) (similar failure)
  'Test'
• trunk / macos-py3-arm64 / test (default, 3, 3, macos-m1-stable) (gh) (disabled by #131103)
  inductor/test_compiled_autograd.py::TestCompiledAutograd::test_autograd_cpp_node

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[albanD]

Thanks a lot for taking the time to fix this!

FYI @XuehaiPan

[albanD]

This is imported by side effect by "import torch" so this is not needed today

[zou3519]

@pytorchbot merge -f "rocm infinite queue"

[pytorchmergebot]

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