[Job] Support DAG execution by replacing `is_chain` with `is_dag` check

[andylizf]

Replace is_chain() with is_dag() check to:

1. Enable future DAG execution support

2. Validate DAG structure before job launch

          Also, Lets create another PR to check the inputt graph is a connected DAG?
   

Originally posted by @cblmemo in https://github.com/skypilot-org/skypilot/pull/4128#discussion_r1817400139

Tested (run the relevant ones):

• [x] Code formatting: bash format.sh
• [ ] Any manual or new tests for this PR (please specify below)
• [ ] 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