[BugFix] Changing the dm_control import to fail if not installed (#515)

* Changing the dm_control import to fail if not installed. Initial commit

* Moving import check to init of the object

* Cleaning up initial commit

Author: zeenolife

torchrl/envs/libs/dm_control.py

@@ -272,6 +272,11 @@ class DMControlEnv(DMControlWrapper):
     """
 
     def __init__(self, env_name, task_name, **kwargs):
+        if not _has_dmc:
+            raise ImportError(
+                "dm_control python package was not found."
+                "Please install this dependency."
+            )
         kwargs["env_name"] = env_name
         kwargs["task_name"] = task_name
         super().__init__(**kwargs)