run LitServe with minimal dependency

[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?

After this PR, it won't be required to install PyTorch to run LitServe.

Also, adds test to make sure LitServe doesn't leak any extra requirement.

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

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79%. Comparing base (4a3721c) to head (dcc145a).

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #138   +/-   ##
===================================
- Coverage    80%    79%   -0%     
===================================
  Files        13     13           
  Lines       867    870    +3     
===================================
+ Hits        690    691    +1     
- Misses      177    179    +2     

[lantiga]

Looks great, one suggestion to simplify the test

[aniketmaurya]

@Borda could you please help review so that we can merge?