CI/PYTEST fully implemented + Optimizers improvement

[doxav]

1. New PYTEST/CI fully implemented

• CI uses Phi4-mini with ollama to run additional tests for optimizers with real LLM (moved tests requiring real LLM out of unit_tests to llm_optimizers_tests)

2. I converted all the unit tests to pytest (101 tests - it will just skip visualization if graphviz is not installed)
3. All real LLM tests are into a separate test folder tests/llm_optimizers_tests. Only test_optimizer.py will run for CI for over each optimizer but continue CI on error. I also created a BBH test (should be customized) and OptoPrimeMulti.
4. Please CHECK OptoPrime modification: tests on different LLM highlighted a frequent confusions between #Variables, #Input and #Code for trainable params; I propose a new version of construct_update_dict to better fallback, you might disagree
5. LLM class properly handle AutoGen / LiteLLM / CustomLLM
6. I inserted a section into README.MD to explain a little bit (I will add more following our exchanges)
7. Minor fixes (TextGrad LiteLLM issue, setup.py)

[chinganc]

@doxav Thanks for contributing this PR!! I was just about to do the pytest migration thing too.

[chinganc]

@doxav Mostly look good to me. Can you separate the changes to optoprime.py into a separate PR? I think the rest should be mergeable once I do some tests. We're planning to do an overhaul of optoprime and it will be cleaner to have a separate thread on that. Thanks!