- Refactor observation encoder in modeling_sac.py

- added `torch.compile` to the actor and learner servers.
- organized imports in `train_sac.py`
- optimized the parameters push by not sending the frozen pre-trained encoder.

Co-authored-by: Adil Zouitine <adilzouitinegm@gmail.com>

lerobot/common/policies/sac/configuration_sac.py

@@ -55,9 +55,10 @@ class SACConfig:
     )
     camera_number: int = 1
     # Add type annotations for these fields:
-    vision_encoder_name: str = field(default="microsoft/resnet-18")
+    vision_encoder_name: str | None = field(default="microsoft/resnet-18")
+    freeze_vision_encoder: bool = True
     image_encoder_hidden_dim: int = 32
-    shared_encoder: bool = False
+    shared_encoder: bool = True
     discount: float = 0.99
     temperature_init: float = 1.0
     num_critics: int = 2