Improve unit tests

[mrwyattii]

As part of an effort to reduce the overall unit test time of DeepSpeed repositories in microsoft/DeepSpeed#3829.

Reduced UT time from ~37m to 25m

• Refactor unit tests:
  • Move shared fixtures to conftest.py to follow pytest best practices
  • Split tests into multiple files with more descriptive names
• Consolidate fixtures for deployment types
  • Added a new deployment_type fixture that allows easy switching of the deployment type in the deployment fixture
• Reduce number of tests being done
  • MII unit tests were initially testing DeepSpeed-inference, however DS-inference has unit tests now on the DeepSpeed repo. This means that we don't need to test all the functionality of DS-inference, just the MII functionality here
• Add coverage report Adding a coverage report is not straightforward because of how we spawn and terminate subprocesses with MII. Will need to look into this further.
• Consolidate CPU-only and GPU tests to just run on self-hosted runners (overhead for CPU-only tests was excessive for how short the CPU tests are)
• Add @pytest.mark.deepspeed for indicating which tests DeepSpeed CI should run