refactor: made platforms a constant

src/qibojit/backends/__init__.py

@@ -3,13 +3,16 @@
 from qibojit.backends.cpu import NumbaBackend
 from qibojit.backends.gpu import CupyBackend, CuQuantumBackend
 
+QibojitBackend = NumbaBackend | CupyBackend | CuQuantumBackend
+
 
 class MetaBackend:
  """Meta-backend class which takes care of loading the qibojit backends."""
 
  PLATFORMS = ("numba", "cupy", "cuquantum")
 
- def load(self, platform: str) -> NumbaBackend | CupyBackend | CuQuantumBackend:
+ @staticmethod
+ def load(platform: str) -> QibojitBackend:
  """Loads the backend.
 
  Args:
@@ -27,15 +30,15 @@ def load(self, platform: str) -> NumbaBackend | CupyBackend | CuQuantumBackend:
  else:
  raise_error(
  ValueError,
- f"Unsupported platform, please use one among {self.platforms}.",
+ f"Unsupported platform, please use one among {PLATFORMS}.",
  )
 
  def list_available(self) -> dict:
  """Lists all the available qibojit backends."""
  available_backends = {}
- for platform in self.platforms:
+ for platform in PLATFORMS:
  try:
- self.load(platform=platform)
+ MetaBackend.load(platform=platform)
  available = True
  except:
  available = False