[fix] Assign CUDAEvent external member properly

[guangyey]

Stack from ghstack (oldest at bottom):

• Move CUDAEvent to c10 #158219
• -> [fix] Assign CUDAEvent external member properly #167711

Motivation

This PR aims to fix the bug that the moved-to object's external_ member is not assigned correctly.

Additional Context

It's not fine to swap the valid value and the invalid value. We'd just need to prevent double-free.

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 7bcf0a4 with merge base 0dac408 ():

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• trunk / linux-jammy-py3-clang12-executorch / test (executorch, 1, 1, lf.linux.2xlarge, unstable) (gh) (#166072)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_int8_static_quant_recipe

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[guangyey]

@ezyang @albanD this is a bug fix. Could you help take a look at.

[albanD]

Any way we can have a test for this?