[Core] Add L4 to accelerator_registry

[romilbhardwaj]

Adds L4 to accelerator registry. This allows us to bypass unnecessary calls to service_catalog.list_accelerators every time resource.copy() is called.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Tested sky launch --gpus L4:1

[concretevitamin]

(comment for the future since we're on a time crunch)

Should L8-19 be updated to reflect this? Basically, when to add an accelerator to this list, the consequence of adding / omitting. Better yet if we could do this automatically.