text: Title: Using `sky launch -t <gpu_instance_type>` without specifying `--gpus` doesn't work
  Body: Repro
  ```
  (py310) sky launch --num-nodes 10 -t g4dn.12xlarge --use-spot --down -i0
  sky.exceptions.ResourcesMismatchError: Infeasible resource demands found:
    Instance type requested: g4dn.12xlarge
    Accelerators for g4dn.12xlarge: {'T4': 4}
    Accelerators requested: None
  To fix: either only specify instance_type, or change the accelerators field to be consistent.
  (py310) sky launch --num-nodes 10 -t p3.2xlarge --use-spot --down -i0                                                                1 ↵
  sky.exceptions.ResourcesMismatchError: Infeasible resource demands found:
    Instance type requested: p3.2xlarge
    Accelerators for p3.2xlarge: {'V100': 1}
    Accelerators requested: None
  To fix: either only specify instance_type, or change the accelerators field to be consistent.
  ```
  commit: 9d1803ba69583a558a3c2e73418c1b3f5590da3b
  
  Don't remember if this is a deliberate choice we made. It's a surprising behavior to me.