Fix/bug fixes langtest 2 1 0 rc1

[chakravarthik27]

This pull request addresses several bugs:

Documentation:

• Updated paths: Image and URL paths in the README.md file have been corrected to ensure proper linking.

Multi-model Support:

• Default value: A default value of False has been added for the __is_multi_model variable in the Harness class to handle multi-model scenarios.
• Test cases: Edited test cases have been added for both multi-dataset and single-dataset configurations. Comments have been included in the imported test cases for clarity in the single-dataset case.

Other Fixes:

• Dataset name fix: The dataset_name variable in datasource.py has been fixed to reflect its correct usage.
• Default folder: The default .langtest folder path has been corrected in the UserProfile Path.
• Typo fix: A typo in the get_embedding method call has been rectified.
• Summarization: A server_prompt parameter has been added to the run method for the summarization task.