Do not attempt importing submodules if framework is not available

[timmoon10]

Description

https://github.com/NVIDIA/TransformerEngine/pull/839 breaks Lightning Thunder's JIT infrastructure (see https://github.com/NVIDIA/TransformerEngine/pull/839#issuecomment-2108824717). Thunder calls inspect.stack, which indiscriminately interacts with all of Transformer Engine's submodules, including lazily loaded submodules for JAX and PaddlePaddle that are intended to throw import errors.

This is a quick fix that only attempts to import submodules if the corresponding DL framework is available. The original bug may reemerge if a Thunder user incidentally has JAX or PaddlePaddle in their Python environment.

Type of change

• [ ] Documentation change (change only to the documentation, either a fix or a new content)
• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Changes

• Do not attempt importing submodules if framework is not available

Checklist:

• [x] I have read and followed the contributing guidelines
• [ ] The functionality is complete
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes