[Core] Add L4 to accelerator_registry (skypilot-org#2724)

* Add L4

* add note

sky/utils/accelerator_registry.py

@@ -12,7 +12,10 @@
 # The list is simply an optimization to short-circuit the search in the catalog.
 # If the name is not found in the list, it will be searched in the catalog
 # with its case being ignored. If a match is found, the name will be
-# canonicalized to that in the catalog.
+# canonicalized to that in the catalog. Note that this lookup can be an
+# expensive operation, as it requires reading the catalog or making external
+# API calls (such as for Kubernetes). Thus it is desirable to keep this list
+# up-to-date with commonly used accelerators.
 # 3. (For SkyPilot dev) What to do if I want to add a new accelerator?
 # Append its case-sensitive canonical name to this list. The name must match
 # `AcceleratorName` in the service catalog, or what we define in
@@ -37,6 +40,7 @@
     'P40',
     'Radeon MI25',
     'P4',
+    'L4',
 ]