Update cxxopts to v3.3.1

[vishesh9131]

Description of the change

Configuring the project with CMake 4.x fails inside extern/cxxopts:

CMake Error at extern/cxxopts/CMakeLists.txt:20 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

cxxopts v2.2.0 declares cmake_minimum_required(VERSION 3.1). This updates it to v3.3.1 (the latest release, which declares 3.5...3.19), so the project configures on CMake 4 without passing -DCMAKE_POLICY_VERSION_MINIMUM=3.5. The CXXOPTS_BUILD_EXAMPLES, CXXOPTS_BUILD_TESTS and CXXOPTS_ENABLE_INSTALL options set in extern/CMakeLists.txt still exist in v3.3.1.

Only the image captioning tutorial uses cxxopts, and it needs no source changes (v3's Options::parse takes const char* const*, which char** converts to).

Testing (CMake 4.4.3, GCC 11.4, Ubuntu 22.04, libtorch 2.10.0): configuring with DOWNLOAD_DATASETS=OFF and building image-captioning succeed without the policy flag. I compared the old and new binaries:

Check	v2.2.0	v3.3.1
--help	48 lines	48 lines (same text, trailing whitespace on wrapped lines)
default options	Vocabulary size: 4076	Vocabulary size: 4076
--min_word_frequency=5 --batch_size=8 --validate_on_epoch_end=false	Vocabulary size: 2985	Vocabulary size: 2985
unknown option / malformed value	uncaught option_not_exists_exception / argument_incorrect_type	uncaught exceptions::no_such_option / exceptions::incorrect_argument_type

Type Of Change

• Bug Fix (non-breaking change that fixes an issue)
• New Feature
• New PyTorch tutorial
• Breaking Change (cmake changes, fix or feature that would cause existing functionality to not work as expected)

Related Issues

None; found while building the tutorials with CMake 4 (no existing issue).

Development & Code Review

• cpplint rules passes locally (run cmake -P cpplint.cmake)
• CI is passing
• Changes have been reviewed by at least one of the maintainers

[prabhuomkar]

why didn't the CI catch it? anyway we can handle this on CI side as well?