[C++ API] Fix ModuleList compile error: error: 'begin' was not declared in this scope

[yf225]

One example in the current docs for torch::nn::ModuleList doesn't compile, and this PR fixes it.
Fixes #32414.

[dr-ci]

💊 CircleCI build failures summary and remediations

As of commit 75a5d44 (more details on the Dr. CI page):

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✅ None of the build failures appear to be your fault 💚

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• 1/1 broken upstream at merge base 7d9f611 since Mar 07

  Please rebase on the viable/strict branch (expand for instructions)

  Since your merge base is older than viable/strict, run these commands:

  git fetch origin viable/strict
  git rebase viable/strict
  

  Check out the recency history of this "viable master" tracking branch.

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🚧 1 upstream failure recognized by patterns:

These were probably caused by upstream breakages:

• pytorch_linux_xenial_py3_clang5_asan_test from Mar 07

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[facebook-github-bot]

@yf225 merged this pull request in 739d460.