add llama3 example

[aniketmaurya]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fixes # (issue).

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #51 (f0f9106) into main (9f0c054) will not change coverage.
Report is 7 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@        Coverage Diff         @@
##           main   #51   +/-   ##
==================================
  Coverage    82%   82%           
==================================
  Files         5     5           
  Lines       296   296           
==================================
  Hits        242   242           
  Misses       54    54           

[aniketmaurya]

Waiting for Lightning-AI/litgpt#1329