[Optimizer] List GPU Instances When a Large Number of CPUs are Requested

[justin-e-dong]

Closes: #3597

This change adds logic to search for a matching GPU instance if there is no CPU instance that satisfies the requirements.

Tested (run the relevant ones):

• 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

Manual Tests:

$ sky launch --cpus 192+
I 09-23 19:11:00 optimizer.py:719] == Optimizer ==
I 09-23 19:11:00 optimizer.py:730] Target: minimizing cost
I 09-23 19:11:00 optimizer.py:742] Estimated cost: $5.3 / hour
I 09-23 19:11:00 optimizer.py:742] 
I 09-23 19:11:00 optimizer.py:867] Considered resources (1 node):
I 09-23 19:11:00 optimizer.py:937] -------------------------------------------------------------------------------------------------
I 09-23 19:11:00 optimizer.py:937]  CLOUD   INSTANCE           vCPUs   Mem(GB)   ACCELERATORS   REGION/ZONE     COST ($)   CHOSEN   
I 09-23 19:11:00 optimizer.py:937] -------------------------------------------------------------------------------------------------
I 09-23 19:11:00 optimizer.py:937]  AWS     m6a.48xlarge       192     768       -              ap-south-1      5.33          ✔     
I 09-23 19:11:00 optimizer.py:937]  GCP     n2d-standard-224   224     896       -              asia-south1-a   6.25                
I 09-23 19:11:00 optimizer.py:937] -------------------------------------------------------------------------------------------------
I 09-23 19:11:00 optimizer.py:937]