[Example] Distributed nccl test with OpenMPI

[Michaelvll]

In our example, there is no OpenMPI-based way to start distributed programs. This PR adds an example for it on GCP.

There are several issues discovered:

• OpenMPI/NCCL may be installed in different paths on various clouds' base image, making the task YAML writing a painful experience.
• We do not have SSH config set up for head node to access the worker node, making it hard for setting up the hostfile OpenMPI (in this example, we hack it by uploading the private key ~/.ssh/sky-key to the remote cluster -- not secure). [Core] Allow ssh access from head to worker nodes #3690

TODO:

• Make this example runnable on any cloud.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • sky launch -c nccl-test --cloud gcp --use-spot examples/distributed_nccl_test_with_mpi.yaml
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh