Updated oumi infer to support CLI argument for system prompt

[Spaarsh]

Description

This change adds an optional argument for system_prompt in the CLI for the interactive mode. The two tests have been added that test this change with and without image as an input.

The change specifically adds a new argument in infer and infer_interactive functions. It also adds new Message object in the Conversation object for inputs with and without an image input.

Further, since this added new code, tests have been added to ensure their functioning. Two tests have been added with and without image input specifically to decouple the testing of system prompt feature independent of the image input for future triages!

Related issues

Fixes #1386

Before submitting

• This PR only changes documentation. (You can ignore the following checks in that case)
• Did you read the contributor guideline Pull Request guidelines?
• Did you link the issue(s) related to this PR in the section above?
• Did you add / update tests where needed?

Reviewers

At least one review from a member of oumi-ai/oumi-staff is required.

[nikg4]

@Spaarsh some unit tests updates may be needed to take care of the new argument. Please go to the following dir and verify that all CLI unit tests pass:

cd tests/unit/cli/
pytest -s -vv

https://github.com/oumi-ai/oumi/blob/main/CONTRIBUTING.md#running-tests

[Spaarsh]

@Spaarsh some unit tests updates may be needed to take care of the new argument. Please go to the following dir and verify that all CLI unit tests pass:

cd tests/unit/cli/
pytest -s -vv

https://github.com/oumi-ai/oumi/blob/main/CONTRIBUTING.md#running-tests

Yep it was a mistake from my part. Still figuring out the tests.😅

I have fixed them now, along with the other changes that you suggested. Thanks!

[ciaralema]

Hey @Spaarsh - thank you for doing this! We'd love to chat to get your quick feedback - would you mind pinging me in Discord? My handle is @ ciara_oumi