[RLlib] Fix erroring our when tuning any algo without torch being ins…

…talled. (ray-project#31177)

rllib/algorithms/qmix/qmix_policy.py

@@ -22,7 +22,7 @@
 from ray.rllib.utils.torch_utils import apply_grad_clipping
 
 # Torch must be installed.
-torch, nn = try_import_torch(error=True)
+torch, nn = try_import_torch(error=False)
 
 logger = logging.getLogger(__name__)
 
@@ -167,6 +167,12 @@ class QMixTorchPolicy(TorchPolicy):
  """
 
  def __init__(self, obs_space, action_space, config):
+ # We want to error out on instantiation and not on import, because tune
+ # imports all RLlib algorithms when registering them
+ # TODO (Artur): Find a way to only import algorithms when needed
+ if not torch:
+ raise ImportError("Could not import PyTorch, which QMix requires.")
+
  _validate(obs_space, action_space)
  config = dict(ray.rllib.algorithms.qmix.qmix.DEFAULT_CONFIG, **config)
  self.framework = "torch"