fix: make multistorageclient import conditional in async_writer example

[ShreyaGupta08]

The async_writer.py example unconditionally imports multistorageclient, which causes an immediate ImportError for users who don't have it installed — even when they're not using the --enable_msc flag.

This change makes the import conditional (try/except) and adds an explicit guard in main() that raises a clear error message only when --enable_msc is actually requested without the package installed.

[greptile-apps]

Greptile Summary

Wrapped the multistorageclient import in a try/except block and added a runtime check that raises a clear error message only when --enable_msc is used without the package installed.

• Import is now conditional (sets msc = None on ImportError)
• Guard added in main() to validate msc is available when --enable_msc flag is set
• All existing usages of msc are already protected by if enable_msc checks
• Follows existing codebase patterns for handling optional dependencies

Confidence Score: 5/5

• This PR is safe to merge with no issues found
• The change is simple, well-targeted, and follows established patterns in the codebase. All usages of msc are already protected by conditional checks, and the new guard ensures a clear error message is shown to users who need the package
• No files require special attention

Important Files Changed

Filename	Overview
examples/checkpointing/async_writer.py	Made multistorageclient import conditional to prevent ImportError when package is not installed and --enable_msc flag is not used

_{Last reviewed commit: f063c89}

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[hexinw-nvidia]

@ShreyaGupta08 Please sign your commit. Thanks.
https://github.com/NVIDIA/nvidia-resiliency-ext/blob/main/CONTRIBUTING.md#signing-your-work

[greptile-apps]

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[ShreyaGupta08]

@hexinw-nvidia all the tests pass, can you merge the PR? I don't get an option to merge. Thanks!