[C++ API] Fix ModuleList compile error: error: 'begin' was not declared in this scope (#34463)
Summary:
One example in the current docs for `torch::nn::ModuleList` doesn't compile, and this PR fixes it.
Fixes https://github.com/pytorch/pytorch/issues/32414.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34463
Test Plan: Imported from GitHub, without a `Test Plan:` line.
Differential Revision: D20331120
Pulled By: yf225
fbshipit-source-id: 50bb078fe1a900c9114d5434e92dc40ee13b52bf
