Don't check acquire the eval context's lock in DALI's background thread

Signed-off-by: Rostan Tabet <rtabet@nvidia.com>

Author: rostan-t

dali/python/nvidia/dali/experimental/dynamic/_eval_context.py

@@ -210,7 +210,7 @@ def _is_current(self) -> bool:
         return self is _tls.default.get(current_device_id)
 
     def __enter__(self):
-        if not self._lock.acquire(blocking=False):
+        if not self._is_in_background_thread() and not self._lock.acquire(blocking=False):
             raise RuntimeError("An EvalContext cannot be active in two threads simultaneously.")
         try:
             _tls.stack.append(self)
@@ -235,7 +235,8 @@ def __exit__(self, exc_type, exc_value, traceback):
             if self._device:
                 self._device.__exit__(exc_type, exc_value, traceback)
         finally:
-            self._lock.release()
+            if not self._is_in_background_thread():
+                self._lock.release()
 
     def evaluate_all(self):
         """Evaluates all pending invocations."""